GI First Aid Q&A Step 1
1. A 35-year-old woman who is HIV positive pre- sents to the physician because of jaundice and right upper quadrant abdominal pain. She re- ports having had multiple episodes of jaundice over the past 10 years. Physical examination is remarkable for scleral icterus, marked ascites, and splenomegaly. A hepatitis panel is positive for HBsAg and anti-HBc IgM, but negative for HBsAb and anti-HAV IgM. Which of the fol- lowing would most likely be lower than the normal reference range in this patient?
1. The correct answer is C. This patient has a flare-up of her chronic hepatitis B virus (HBV) infection, as evidenced by the presence of HBsAg and anti-HBc IgM and lack of HBsAb. HBV typically presents with jaundice and right upper quadrant pain and can be transmit- ted via parenteral, sexual, and maternal-fetal routes. About 5% of adults with acute HBV infection will develop chronic hepatitis, and 12%-20% of these will go on to develop cirrho- sis. Chronic HBV infection is marked by the presence of HBsAg for >6 months. While most patients will develop HBsAb and eliminate HBsAg from the blood, chronically infected patients do not. A patient with HIV may have a history of risky sexual behavior and would be at increased risk for HBV infection; in fact, chronic HBV infection affects about 10% of HIV-infected patients worldwide. This patient's signs of scleral icterus, ascites, and spleno- megaly indicate that her chronic liver disease may have progressed to cirrhosis. The fibrotic liver can induce portal hypertension, causing engorgement of the spleen due to increased pressures within the portal circulation. This enlarged spleen sequesters increased numbers of platelets within it. Thus in this patient we might expect a low platelet count.
10. A 16-year-old boy presents to the clinic with "skin boils." The lesions are erythematous and tender, concerning for skin abscesses due to methicillin-resistant Staphylococcus aureus. The patient is promptly given an oral medica- tion to treat the infection. The abscesses even- tually resolve. Three days later, the boy devel- ops fever and watery, foul-smelling diarrhea. What is the mechanism of action of the antibi- otic the patient is taking?
10. The correct answer is B. Methicillin-resistant Staphylococcus aureus (MRSA) is an organ- ism that is resistant to traditional penicillin family of antibiotics. Therefore, MRSA must be treated with clindamycin, trimethoprim- sulfamethoxazole, or vancomycin. Watery, foul-smelling stool in the presence of fever fol- lowing antibiotic treatment usually is caused by Clostridium difficile superinfection. Of these various therapies for MRSA, clindamy- cin is the only one that is known to cause a C difficile colitis from antibiotic-induced bac- terial overgrowth. Clindamycin disrupts bac- terial protein synthesis by blocking the 50S subunit of the ribosome. It is used frequently to treat anaerobic infections and has proven ef- ficacy against MRSA infections. In this patient, clindamycin would have been a good choice for treatment of a community acquired MRSA infection.
27. A 26-year-old man with hepatitis C is be- ing treated medically while he awaits liver transplantation. One of the drugs he is tak- ing causes him to have periodic fevers and chills and a sense of depression that he did not have prior to treatment. Which of the follow- ing drugs most likely is responsible for this pa- tient's adverse effects?
27. The correct answer is C. Pegylated inter- feron is a cytokine derivative that improves the body's antiviral response. It is used in the treatment of HBV and HCV. Adverse effects of interferon therapy include a flu-like reac- tion that manifests as episodic fevers and chills, as well as occasional profound depression. As a result, interferon is contraindicated in se- verely depressed or suicidal patients. Although interferon is not a cure for hepatitis, it is rec- ommended to slow the progression of cirrhotic liver disease in some patients. Pegylated inter- feron is a longer-acting form of interferon.
11. A 65-year-old woman presents to the emer- gency department with persistent right upper quadrant pain with nausea and vomiting. CT of the abdomen reveals a polypoid mass of the gallbladder protruding into the lumen, diffuse thickening of the gallbladder wall, and en- larged lymph nodes. This patient most likely has a history of which of the following?
11. The correct answer is C. The patient's clini- cal presentation is consistent with adenocar- cinoma of the gallbladder. Gallbladder ad- enocarcinoma is associated with chronic gallbladder inflammation, typically from a history of gallstones, which can be seen with the thickening of the gallbladder wall on CT. Gallbladder polyps, the polypoid lesion, are also associated with an increased risk of gall- bladder adenocarcinoma. The enlarged lymph nodes point to local invasion and spread, which is unfortunately common on initial pre- sentation. Gallbladder cancer is a disease of the elderly and is more common in women than men. Most (90%) patients with gallblad- der cancer have concomitant stones. In gen- eral, the treatment for adenocarcinoma of the gallbladder is surgical excision but prognosis is generally poor if not found incidentally.
12. A 35-year-old woman presents to the emer- gency department because of abdominal pain and diarrhea mixed with mucus and blood. She also has ulcerated lesions with violaceous borders on her legs. Gross blood is present on rectal examination. A biopsy of her colon re- veals inflammation confined to the mucosa and submucosa, as shown in the image. Which of the following would most likely be used to treat this patient?
12. The correct answer is F. The patient has ul- cerative colitis. The leg lesions represent pyo- derma gangrenosum and are the first clue of an extra-intestinal manifestation of ulcerative colitis. The diagnosis is confirmed with the biopsy showing that the inflammation is con- tained to the mucosal and the submucosal layers (remember that in Crohn disease the inflammation is transmural, leading to fistula formation). Sulfasalazine is a combination of sulfapyridine, which is an antibacterial drug, and mesalamine, which is an anti-inflamma- tory drug. Its adverse effects include malaise, nausea, sulfonamide toxicity, and reversible oligospermia. Immunosuppressive drugs such as 6-mercatopurine and methotrexate can be used to treat ulcerative colitis and Crohn dis- ease.
13. A fourth-year medical student is working in a medical relief group in Haiti for several months. Several parents bring their children to the clinic and explain that the children have had profuse, watery stool along with wa- tery vomiting. All of the children are afebrile, slightly hypotensive, and tachycardic but have a normal respiratory rate. Urine output is re- duced. What would be the best immediate management of this diarrhea?
13. The correct answer is E. The presentation and history suggest cholera. Cholera is caused by Vibrio cholerae, a gram-negative comma- shaped bacterium. It can cause profuse secre- tory diarrhea with watery vomiting in some patients. It has been a problem in many parts of the world, especially after natural disasters, as mortality in untreated patients exceeds 50%. Initial management in the setting of moder- ate or mild dehydration secondary to diarrhea is oral rehydration therapy (ORT). The pre- sentation of these patients suggests moderate dehydration. ORT consists of administering glucose-containing sodium solution in a ratio not exceeding two glucose molecules per one sodium molecule. Dehydration is prevented by shifting fluid from the intestinal lumen into the circulation, secondary to glucose-coupled sodium transport in the mucosal cells.
14. A 79-year-old woman presents with 5.4-kg (12 lb) weight loss over the past two months, asso ciated with progressively worsening dull, con stant abdominal pain, early satiety, and nausea Her examination is notable for a palpable periumblicial node as well as left supraclavicu lar adenopathy. Which of the following is the most likely diagnosis?
14. The correct answer is B. Patients with gastric cancer tend to present with abdominal pain, anorexia, early satiety, nausea, and/or dyspha- gia. Weight loss tends to be secondary to in- sufficient food intake, but may also be due to gastric stasis or outlet obstruction. The abdom- inal pain progressively worsens as the disease spreads. Lymphatic spread is common and re- sulting examination findings may show a peri- umbilical nodule (Sister Mary Joseph's node) as well as left supraclavicular adenopathy (Vir- chow node).
15. A medical student presents to an infectious dis- ease specialist complaining of abdominal dis- tention and tenderness. The patient reports no recent changes in normal bowel habits. Physi- cal examination shows hepatosplenomegaly. Bowel sounds are normal. On questioning, the patient reports that he traveled to Brazil several months ago to study tribal medical practices. He frequently went swimming in the Amazon River to wash himself. Several weeks after re- turning from his trip, he recalls having fever, diarrhea, weight loss, and "funny looking" stools. Which of the following conditions is most likely responsible for this patient's present symptoms?
15. The correct answer is D. Schistosomiasis is a parasitic disease with hepatic involvement. Schistosoma mansoni larva, which are com- monly found in fresh waters of South America, penetrate the host's skin, invade the peripheral vasculature, and eventually settle in the portal or pelvic venous vasculature. Several weeks following infection, patients may develop symptoms similar to the ones described, such as fever, diarrhea, and weight loss; the "funny looking" stools likely represent S mansoni eggs. Chronic infection may eventually lead to por- tal hypertension and hepatosplenomegaly, leading in turn to ascites and eventually cirrho- sis. In addition, the hepatosplenomegaly leads to esophageal varices, producing bleeding that can often be the first clinical sign.
17. A 20-year-old man has Crohn disease that is refractory to treatment with high-dose meth- ylprednisolone. He is started on therapy with infliximab, a chimeric monoclonal antibody with anti-inflammatory effects. This drug is administered intravenously every two months and produces substantial improvement in the patient's symptoms between doses. Which of the following best describes infliximab's mech- anism of action?
17. The correct answer is B. Recent studies have demonstrated the benefit of infliximab (Remi- cade) in the treatment of Crohn disease that is refractory to steroid treatment. It also is ap- proved for use in a variety of other autoim- mune diseases such as ulcerative colitis, an- kylosing spondilitis, psoriasis, and psoriatic arthritis. Infliximab is a monoclonal chimeric antibody that binds soluble tumor necrosis factor-a (TNF-a), and as a result blocks its ef- fects. TNF-a is a pro-inflammatory cytokine secreted by macrophages that is found in high concentrations in the stool of Crohn patients. The chimeric antibody is 75% human and 25% murine. A single infusion produces a clinical response in 65% of patients. Common adverse effects are increased susceptibility to upper respiratory infections, headache, and GI distress.
18. A 2-month-old boy is brought to his pediatri- cian for a regular check-up. His parents report that he has a poor appetite and is very consti- pated. He has small bowel movements once a week, which his parents say appear to be very painful. Although he was at the 75th percen- tile for both height and weight at birth, he is currently at the 25th percentile for height and is below the fifth percentile for weight. His ab- domen is distended, but his bowel sounds are normal and his abdomen does not appear to be tender. Barium enema shows a narrow rec- tosigmoid with a dilation of the segment above the narrowing, and a rectosigmoid biopsy shows a conspicuous absence of ganglion cells. Which of the following genetic conditions is most commonly associated with this patient's disease?
18. The correct answer is B. This patient suffers from Hirschsprung disease, which manifests when neural crest cells fail to migrate to the distal colon. Consequently, enteric neurons do not form in a segment of the rectosigmoid; these neurons are normally responsible for re- laxation of the rectum to allow defecation. If this condition is left untreated, these infants run the risk of developing enterocolitis or a bowel perforation. Ten percent of cases of Hirschsprung disease occur in children with Down syndrome, caused by trisomy 21. Chil- dren with Down syndrome also have an in- creased risk of duodenal atresia, congenital heart disease, and acute lymphoblastic leuke- mia.
19. An 8-year-old boy presents to the emergency department because of 18 hours of severe vom- iting. Arterial blood gas analysis reveals a pH of 7.48, a bicarbonate level of 35 mEq/L, and a partial carbon dioxide pressure of 48 mm Hg. Which of the following best describes the acid- base disturbance occurring in this patient?
19. The correct answer is F. This patient is pre- senting with only slight alkalemia (normal arterial pH is 7.35-7.45). However, the bicar- bonate level is substantially elevated, about 11 mEq/L above normal. This implies that a met- abolic alkalosis is occurring, which can be ex- plained by the patient's recent history of severe protracted vomiting. Vomiting causes a loss of hydrochloric acid from the GI tract; this acid must be replaced, which is done by drawing hydrogen from body stores and leaving bicar- bonate behind. Normally with this level of bi- carbonate elevation alone, a higher pH would be expected. However, this patient's partial car- bon dioxide pressure (PCO2) is elevated to 48 mm Hg. This implies that a normal respiratory compensation has occurred in order to nor- malize the pH by retaining acid by the forma- tion of carbonic acid from carbon dioxide. The expected compensation is an increase of 0.7 mm Hg of carbon dioxide for every 1-mEq/L increase in bicarbonate. This patient's bicar- bonate level has increased by about 11 mEq/L, and the PCO2 has increased by about 8 mm Hg, an appropriate compensation.
2. An 85-year-old woman presents to the emer- gency department because of sudden onset of abdominal pain, maroon-colored stools, and abdominal distention. She denies any past ab- dominal surgery. An upper gastrointestinal fluoroscopy study is performed with the results shown below. Which of the following is the most likely cause of this patient's symptoms?
2. The correct answer is E. This is an example of volvulus, a twisting of the large intestine in a closed-loop obstruction. The elderly and debil- itated are at a particular risk for volvulus. This patient has the common symptoms of colonic obstruction, with abdominal pain, abdomi- nal distention, and bloody stools. The image shows the nonspecific "double-bubble," which is a sign of proximal small-bowel obstruction. Colonoscopy frequently is both diagnostic and therapeutic, as insufflations of air and passing the colonoscope through the point of volvu- lus frequently results in reduction of the vol- vulus. The condition frequently recurs, and definitive surgical treatment is a sigmoid co- lectomy, though a sigmoidopexy (fixing of the sigmoid colon to anterior abdominal) can be performed in those patients who are too sick to tolerate an intestinal resection.
20. A 43-year-old multiparous woman with no other medical history presents to her physician because of crampy abdominal pain, fever, and jaundice. Laboratory studies show: Total bilirubin: 4.8 mg/dL Direct bilirubin: 4.2 mg/dL Amylase: 50 U/L Lipase: 70 U/L Aspartate aminotransferase: 75 U/L Alanine aminotransferase: 70 U/L The patient subsequently is sent for endo- scopic retrograde cholangiopancreatography; results are shown in the image. What is the most likely cause of the obstruction seen in the image?
20. The correct answer is A. The elevated direct bilirubin level and the imaging findings are consistent with a common duct obstruction, most likely secondary to a gallstone (consider this woman's risk factors: female, fertile, and >40 years). An obstruction in the common duct (choledocholithiasis) does not allow drainage of bile from the liver or gallbladder, and can lead to cholangitis, which is charac- terized by Charcot's triad of jaundice, fever, and right upper quadrant pain. About 80% of gallstones are made of cholesterol; these oc- cur when solubilizing bile acids and lecithin are overwhelmed by excess cholesterol. The remaining 20% are pigment stones containing mainly calcium bilirubinate; these can occur during periods of increased hemolysis.
21. A 75-year-old woman is taken to the hospital by her son after two bouts of bilious vomiting. Although she normally has a healthy appetite, over the past three days she has had little in- terest in eating. Furthermore, her belly has become rigid and diffusely tender. X-ray of the abdomen reveals dilated loops of small intes- tines. Which of the following predisposes this patient to this condition?
21. The correct answer is C. This patient has a small bowel obstruction. Dilated loops of small intestines on x-ray of the abdomen and a clini- cal history of anorexia, vomiting, and abdomi- nal pain are usually sufficient to make the di- agnosis. In the United States, the leading cause of small bowel obstructions is adhesion forma- tion, which obstructs the lumen of the small bowel. These adhesions are formed during the healing process secondary to abdominal surgery. Other conditions that predispose patients to small bowel obstructions are hernias and in- traluminal cancers of the small intestine.
22. A 27-year-old man presents to his family phy- sician for an annual physical examination. On rectal examination, masses are palpated. The patient is referred for a colonoscopy, which reveals adenomatous polyps located diffusely throughout the colon. When asked about his family history, the patient states that his father passed away from colon cancer. Which of the following inheritance patterns is characteristic of this condition?
22. The correct answer is A. Familial adenoma- tous polyposis (FAP) is an autosomal domi- nant condition characterized by a germline mutation on chromosome 5, specifically at the adenomatous polyposis coli (APC) lo- cus. The APC gene is thought to have tumor- suppressive effects, and its loss is associated with more than colonic cancers. In addition to duodenal neoplasms for which these patients with FAP must undergo lifelong upper en- doscopic surveillance, increased risk exists in these patients for developing gastric, liver, thy- roid, and central nervous system neoplasms.
23. A 10-year-old girl living in Grand Haven, Michigan, is brought to the physician because she has had a fever and headache accompa- nied by vomiting and bloody diarrhea over the last few days. She has no history of recent travel or sick contacts but has a pet puppy, which the mother says has also had diarrhea for the past week. A stool culture incubated at 42°C in a microaerophilic environment shows many gram-negative, comma-shaped organ- isms, each with a single polar flagellum. The organism responsible for this patient's sickness is associated with the possible development of which of the following symptoms?
23. The correct answer is E. Guillain-Barré syn- drome is characterized by rapidly progress- ing ascending paralysis. It is an autoimmune- mediated illness that can occur following a variety of infectious diseases, but is particularly associated with prior infection by Campylo- bacter jejuni. C jejuni gastroenteritis is char- acterized in this patient by her vomiting and bloody diarrhea together with the finding of comma-shaped organisms with a single polar flagellum when cultured at 42°C in a micro- aerophilic environment. Other enteric patho- gens with this morphology include bacteria of the Vibrio genus (V cholera and V parahaemo- lyticus). These species, however, are not en- demic to the United States and would not be expected in a patient without a recent travel history, nor do the symptoms match. C jejuni is transmitted to humans via the fecal-oral route from either domestic animals or eating undercooked poultry.
24. A 2-year-old girl who has recently been adopted from Southeast Asia is brought to the clinic by her adopted parents. They are con- cerned because the child seems to be having trouble with her vision in low-light conditions. The nutrient most likely deficient in this child is absorbed by the gastrointestinal system using what mechanism?
24. The correct answer is C. This patient most likely has vitamin A deficiency, which is char- acterized by early symptoms of night blind- ness, dry conjunctivae, and gray plaques, or late symptoms of corneal ulceration and ne- crosis leading to perforation and blindness. This deficiency is typically seen in children and pregnant women whose diets are deficient in vitamin A. It can also be seen in alcoholics, after intestinal surgery (especially when the il- eum is involved), and in patients with fat mal- absorption, cholestasis, or inflammatory bowel disease. Vitamins A, D, E, and K (fat-soluble vitamins) are absorbed in the small intestine and absorption requires micelles formed with bile salts.
25. A 34-year-old man visits his physician because he has experienced increasing "itchiness" and fatigue over the past three months. Medical history is significant for a total colon resec- tion; pathologic findings at resection are shown in the image. Physical examination reveals scleral icterus. Given his past medical history, ultrasound studies are performed, which re- veal obliteration of the intrahepatic bile ducts. What is this man's most likely underlying con- dition?
25. The correct answer is D. The image shows inflammatory pseudopolyps in a patient with ulcerative colitis. Primary sclerosing cholangi- tis (PSC) is an extraintestinal complication of ulcerative colitis. About 70% of patients with PSC also have ulcerative colitis; however, only about 4% of patients with ulcerative colitis will develop PSC. PSC leads to obliterative fibrosis of intrahepatic and extrahepatic bile ducts, and can over time lead to cirrhosis. Patients with PSC also have an increased risk of developing cholangiocarcinoma.
26. A 29-year-old man complains to his physician of chronic diarrhea. On further questioning he reveals that the diarrhea is watery and in- termittent, and that he also suffers from flatu- lence and weight loss of 3.6 kg (8 lb) over the past year. He denies fever, nausea, vomiting, abdominal pain, and recent travel. Stool ex- aminations for ova and parasites and for occult blood are negative, and stool culture does not grow any pathogens. Endoscopy is performed and biopsy of the upper part of the small intes- tine demonstrates diffuse blunting of villi and a chronic inflammatory infiltrate in the lamina propria. Which therapeutic option will most likely benefit this patient?
26. The correct answer is A. The patient's clinical, laboratory, and histologic features are indica- tive of celiac sprue. The classical presentation of this disease is during infancy, but it may also present any time between the ages of 10 and 40. Classic signs include diarrhea, foul- smelling, bulky, floating stools, weight loss, growth failure, and vitamin deficiencies. These symptoms follow exposure to the protein glia- din, which results in intestinal inflammation. A gluten-free diet usually relieves symptoms and restores mucosal histology, and therefore it is the most appropriate therapeutic measure.
28. A 10-year-old boy is brought to the emergency department by his parents with a low-grade fe- ver, anorexia, nausea, vomiting, and abdomi- nal pain. The parents report that the pain ini- tially began periumbilically and developed into severe right lower quadrant pain after sev- eral hours. On physical examination the child is diaphoretic and lies still; involuntary guard- ing and rebound are present. Pain is elicited when the child is placed on his left side and the right leg is hyperextended against resis- tance. Which of the following provides inner- vation to the muscle involved in this maneu- ver?
28. The correct answer is B. This patient presents with the classic signs and symptoms of appen- dicitis. When the right leg is hyperextended, the iliopsoas muscle group pushes against the appendix and causes significant pain and ir- ritation. Pain with hyperextension will also be present in pancreatic cancers and inflam- mation of the cecum and the sigmoid colon. The psoas muscle is innervated by the lumbar plexus, and the iliacus muscle is innervated by the femoral nerve.
29. A 4-year-old child is brought to the pediatri- cian because of abdominal pain, vomiting, and diarrhea containing mucus and blood. The child has a fever of 39.4°C (103°F). On stool culture, the causative organism is shown to be a non-lactose-fermenting, non-hydrogen sulfide-producing bacterium that is extremely virulent. Which of the following is/are most likely to result from continued infection by this organism?
29. The correct answer is D. Shigella species pro- duce gastroenteritis characterized by abdomi- nal pain, bloody diarrhea, and nausea and/ or vomiting. Additionally, because Shigella species invade intestinal epithelial cells, the illness is accompanied by fever. Shigella is a nonlactose fermenter, and it does not produce gas or hydrogen sulfide. Infection usually af- fects preschool-age children and populations in nursing homes. Transmission occurs by the fecal-to-oral route via fecally contaminated wa- ter and hand-to-hand contact. It's an extremely virulent organism requiring only 10 organisms for infection. Shigella also produces Shiga toxin, which can cleave host rRNA and en- hance cytokine release, resulting in hemolytic uremic syndrome. This syndrome develops af- ter the endothelium is damaged in the kidney and results in renal failure, thrombocytopenia, and microangiopathic hemolytic anemia.
3. A 22-year-old woman with no significant medi- cal history complains of diffuse abdominal pain. Physical examination reveals rebound tenderness in the right lower quadrant. The pa- tient denies being sexually active and has not traveled recently. However, she does mention eating "funny tasting" potato salad at an out- door party three days ago. Which of the follow- ing is the most appropriate next step in man- agement?
3. The correct answer is C. The possibility of ec- topic pregnancy should always be considered in a woman of reproductive age who presents with abdominal pain, regardless of the patient's history. The history of eating potato salad is a distracter, and should not change the clini- cian's decision to measure the b-human cho- rionic gonadotropin (b-hCG). The symptoms of ectopic pregnancy may closely mimic those of acute appendicitis, making measurement of the b-hCG level especially critical in cases of suspected appendicitis. Furthermore, an undiagnosed ectopic pregnancy can result in mor- bidity and even death.
30. A 62-year-old man with a long history of alco- holism presents to the emergency department with steatorrhea and abdominal pain. CT of the abdomen is shown in the image. The in- tern on duty recalls learning about a drug in- dicated for acromegaly that may also reduce the secretion of pancreatic fluids and possibly decrease the patient's pain. The drug works by mimicking the levels of which hormone?
30. The correct answer is D. This patient's symp- toms and pancreatic calcifications on the CT scan are consistent with chronic pancreatitis. The intern is thinking about octreotide, a so- matostatin analog used to treat acromegaly. Among its various actions, somatostatin is a potent inhibitor of growth hormone secre- tion; it also suppresses the release of a number of digestive hormones, such as gastrin, chole- cystokinin, secretin, and vasoactive intestinal peptide (VIP). It also decreases the secretion of pancreatic fluids. By inhibiting the secretion of pancreatic fluids, octreotide may be able to al- leviate this patient's chronic abdominal pain.
31. A 27-year-old woman with no significant medi- cal history complains of a month of sharp, nonradiating, epigastric pain. Her pain is re- lieved after eating food, and she has experi- enced weight gain. What is the most likely pri- mary treatment for this patient?
31. The correct answer is A. This patient is likely suffering from a duodenal ulcer. Helicobacter pylori is the most common cause of duodenal and gastric ulcers (involved in 100% and 70% of lesions, respectively). It can be diagnosed with esophagogastroduodenoscopy or a urease breath test. A key distinction between these two ulcers is that eating food often relieves du- odenal ulcer pain and patients tend to report resulting weight gain. Duodenal ulcer symp- toms are exacerbated when acid is secreted without any food to act as a buffer, causing pain on an empty stomach. The standard first- line therapy is one-week triple therapy consist- ing of the antibiotics amoxicillin and clarithro- mycin, and a proton pump inhibitor such as omeprazole.
32. A 34-year-old woman presents with three weeks of abdominal pain and diarrhea. She says the diarrhea appears to be greasy. She also admits to a lot of flatulence since the gastroin- testinal symptoms began. A stain of the stool is shown in the image. What drug(s) should be used to treat the organism causing her symp- toms?
32. The correct answer is D. The organism caus- ing the patient's symptoms is the protozoan Giardia lamblia. G lamblia trophozoites com- monly cause chronic diarrhea. This parasite is treated with metronidazole. It spreads via oral-fecal transmission in its cyst form and then colonizes the GI tract in its trophozoite form. G lamblia is found primarily in the duode- num and jejunum, and causes a combination of malabsorption with diarrhea through a still incompletely understood mechanism. The pa- tient's complaints of greasy stool and flatulence are classic signs of this type of infection. Diag- nosis is made via direct examination of stool for cysts as well as duodenal fluid sampling and small-bowel biopsy.
33. A 40-year-old man with no significant past medical history presents to the emergency de- partment because of a two day history of fever, vomiting, and diarrhea. His blood pressure is 90/65 mm Hg and pulse is110/min. An intra- venous line is started and he is given 3 L of fluid and then admitted for monitoring. On admission, laboratory studies are unremarkable except for a serum albumin level of 3.0 g/dL. Which of the following is the most likely cause of this patient's laboratory abnormality?
33. The correct answer is A. This patient is most likely suffering from acute gastroenteritis, probably of viral origin. While there are sev- eral causes of hypoalbuminemia, the most likely cause in this otherwise healthy man is simply a dilutional effect due to the large amounts of fluid he was given. This type of hy- poalbuminemia is also seen in congestive heart failure.
34. A 33-year-old man with gastroesophageal re- flux disease returns to his physician for the sec- ond time in two weeks complaining of worsen- ing soreness in his throat. Two weeks earlier he was diagnosed with penicillin-sensitive Strep- tococcus pyogenes on throat culture and was prescribed ciprofloxacin (since he is allergic to penicillin). Review of the patient's medica- tion history reveals a possible drug interaction. Which of the following medications is this pa- tient most likely taking that would reduce the effectiveness of his antibiotic?
34. The correct answer is B. The oral absorption of ciprofloxacin is impaired by divalent cations, including those in common antacids such as calcium carbonate, which this patient is likely taking to treat GERD. Ciprofloxacin belongs to the family of fluroquinolones, which block bacterial DNA synthesis by inhibiting bacterial DNA gyrase. Inhibition of DNA gyrase pre- vents the relaxation of positively supercoiled DNA that is required for normal transcription and replication.
35. A 46-year-old man presents to the emergency department complaining of severe abdominal pain following a weekend of tailgating during which he consumed "a ton" of beer. On physi cal examination the patient has a tempera ture of 38.2°C (100.8°F), with pain located in the epigastric region that periodically radi ates to his back. Laboratory tests show a serum amylase level of 400 U/L and WBC count of 16,000/mm³. What is the most likely compli cation of this disease?
35. The correct answer is D. This patient's presen- tation is classic for acute pancreatitis. This pro- cess often occurs in young patients after con- suming large amounts of alcohol. Other causes include gallstone obstruction, medications, infection, hypertriglyceridemia, and trauma. Pseudocysts often arise after a bout of acute pancreatitis and consist of necrotic, hemorrhagic debris with pancreatic enzymes. These cysts lack a true epithelial lining.
36. A 51-year-old man with a lengthy history of medication-dependent reflux esophagitis sees his physician for an annual physical examina- tion. Laboratory tests reveal a blood gastrin level three times the upper limit of normal. His physician expresses concern that the pa- tient is at risk of developing atrophic gastritis. Which of the following medications is this pa- tient most likely taking?
36. The correct answer is E. Omeprazole is a proton pump inhibitor (PPI) that works by covalently binding, and irreversibly inactivat- ing the H+/K+/ATPase on the luminal surface of the gastric parietal cell. Gastrin levels are regulated by a feedback loop. Intragastric acid- ity stimulates D-cells in the gastric antrum to release somatostatin, which works in a para- crine fashion, binding to G-cells in the gastric antrum and inhibiting gastrin release. PPIs will effectively raise the intragastric pH so that gas- trin levels rise two- to four-fold. Omeprazole is associated with atrophic gastritis due to hy- pergastrinemia. It may also be associated with carcinoid tumors, headaches, and GI distur- bances.
37. A 25-year-old man presents to his primary care physician after several episodes of severe crampy abdominal pain relieved by the pas- sage of loose stool mixed with blood and mu- cus. He says he has been feeling fatigued for the past three months and has lost 6.8 kg (15 lb). He has a fever of 37.3°C (99.1°F); ab- dominal examination is notable for hypoac- tive bowel sounds and diffuse tenderness with guarding. Colonoscopy reveals diffuse, con- tinuous ulcerations of the intestinal mucosa extending proximally from the rectum to the splenic flexure. A diagnosis is made and ge- netic testing reveals the patient carries an HLA subtype commonly associated with the disease. Which other disease is associated with the same human leukocyte antigen subtype?
37. The correct answer is A. This patient suffers from ulcerative colitis (UC), an inflamma- tory bowel disease of unknown etiology that affects rectum and may extend proximally to the colon; disease is rarely found in the small intestine. UC is characterized by mucosal and submucosal inflammation. Friable mucosal pseudopolyps may be evident on colonoscopy. Patients typically present with crampy abdomi- nal pain and bloody diarrhea. UC is associated with the HLA B27 subtype. Diseases associated with the HLA B27 subtype can be remem- bered with the mnemonic PAIR, and include Psoriasis, Ankylosing spondylitis, Inflammatory bowel disease, and Reiter syndrome.
38. A 67-year-old Chinese immigrant with a his- tory of alcohol abuse and chronic hepatitis B virus infection has been experiencing fatigue, weight loss, and vague abdominal pain for sev- eral months. Physical examination reveals a palpable mass in the liver. Before the mass can be surgically resected, the patient dies of respi- ratory failure. The appearance of his liver at autopsy is shown in the image. How does this lesion migrate to other organs in the body?
38. The correct answer is C. This is a case of he- patocellular carcinoma (HCC). As are renal cell carcinoma and follicular thyroid carci- noma, HCC is spread commonly via hema- togenous dissemination. Accordingly, metas- tases often develop in the lung, portal vein, periportal nodes, brain, or bones. The patient's chronic HBV infection and alcohol abuse likely led to the development of cirrhosis and then HCC.
8. A 57-year-old white man is brought to the emergency department by ambulance be- cause of sudden-onset, bright red emesis. His blood pressure is 80/40 mm Hg and heart rate is 124/min. Physical examination is notable for jaundice and an enlarged abdomen that is dull to percussion and positive for a fluid wave. Which of the following vessel anastomoses is responsible for the patient's bleeding?
8. The correct answer is B. The patient's presen- tation is consistent with ruptured esophageal varices, a dangerous complication of portal hy- pertension. When the liver becomes extremely fibrotic, as it does with years of exposure to alcohol (note the jaundice and ascites in this patient), there is an increase in resistance in blood flow through the liver, causing portal hypertension. When the pressure in the portal system is greater than the systemic venous pres- sure, blood will find alternate routes to return to the heart. One of those alternate routes is from the left gastric vein into the azygos vein, which leads to esophageal varices.
39. A 35-year-old man with a history of drinking one-two bottles of vodka per day for the past 15 years presents to the emergency department because of massive hematemesis and severe epigastric pain. He takes antacids to manage mild acid reflux but has no other known medi- cal problems or medications. His temperature is 36.7°C (98.1°F), pulse is 110/min, respira- tory rate is 23/min, and blood pressure is 80/40 mm Hg. Physical examination reveals a regu- lar rate and rhythm with no murmurs and his lungs are clear to auscultation. No jaundice is present. There is no abdominal tenderness or distension, no hepatosplenomegaly, and bowel sounds are present. His stool is negative for blood. Which of the following is the most likely diagnosis?
39. The correct answer is A. This patient has Mallory-Weiss syndrome. Repeated bouts of prolonged vomiting (such as after an alcohol binge or in eating disorders) can cause longi- tudinal lacerations in the distal esophagus, normally at the gastroesophageal junction or in the proximal gastric mucosa, with exten- sion to submucosal arteries that can bleed mas- sively. Left untreated, this bleeding can be fa- tal. Mallory-Weiss syndrome generally presents with hematemesis after a bout of retching or vomiting; however, new research suggests that this classic history may be obtained in only about 50% of patients. Bleeding from esopha- geal varices might also be expected if the pa- tient has chronic liver disease. However, this patient is relatively young and shows no other signs of liver disease. Furthermore, variceal bleeding is usually painless, while Mallory- Weiss tears are more commonly associated with pain. Thus of the two choices, Mallory- Weiss syndrome is the better answer.
4.A 43-year-old overweight woman presents to her doctor's office because of right upper quad- rant abdominal pain. She has experienced sim- ilar episodes of this type of pain in the past and admits that it is worse after meals. Increased se- cretion of which of the following is responsible for this patient's postprandial pain?
4. The correct answer is A. This is a classic pre- sentation of cholelithiasis, or gallstones. Pa- tients with cholelithiasis experience pain after meals as a result of the duodenal release of cholecystokinin (CCK), which causes the gall- bladder to contract while the stone obstruct the cystic duct. CCK is stimulated by fatty ac- ids and amino acids.
40. A 32-year-old woman complains of alternating bouts of diarrhea and constipation and reports chronic abdominal pain relieved by frequent bowel movements. Her symptoms are exac- erbated by stress. The patient denies fever or weight loss. She has a negative fecal occult blood test. Colonoscopy and endoscopy reveal no abnormalities. The most likely diagnosis in this patient is commonly associated with which of the following findings?
40. The correct answer is B. Any biopsy would likely show normal structures. Irritable bowel syndrome is a functional GI disorder charac- terized by abdominal pain and altered bowel habits in the absence of demonstrable organic pathology. It is a diagnosis of exclusion based on clinical features such as the ones presented. Most commonly, patients have alternating di- arrhea and constipation, chronic abdominal pain that improves with stools, a change in stool frequency and consistency, and onset af- ter emotional and/or stressful life events. These symptoms occur in the absence of fevers, lower GI bleeding, leukocytosis, and weight loss.
41. An obese 40-year-old multiparous woman comes to the physician because she has been experiencing right upper quadrant pain with nausea and vomiting precipitated by fatty foods. Results of a right upper quadrant ul- trasound are shown in the image. Laboratory studies show: Total cholesterol: 280 mg/dL LDL cholesterol: 170 mg/dL HDL cholesterol: 33 mg/dL Triglycerides: 420 mg/dL Which of the following drugs is relatively con- traindicated in this patient's treatment?
41. The correct answer is B. The patient has gall- stones, as evidenced by her symptoms and ul- trasound findings of multiple gallstones. She fits the demographics of the classic patient with cholesterol gallstones: Fat, Female, Fer- tile, and Forty (the "4 F's"). Gemfibrozil, a fibrate, is contraindicated in the treatment of hypertriglyceridemia in the presence of gall- stones. Fibrates can increase the development of gallstones, thus increasing the risk of chole- cystitis.
42. A healthy 55-year-old woman presents to the physician with a one-year history of an un- changing, non-painful palpable mass in her left cheek. A parotid gland biopsy reveals groups of well-differentiated epithelial cells in a chondromyxoid stroma surrounded by a fibrous capsule; multiple cell types are visible on light microscopy. The pathologic descrip- tion of the mass is most consistent with which of the following conditions?
42. The correct answer is C. The most common tumor of the parotid gland is the pleomorphic adenoma or the mixed tumor, accounting for 50% of salivary tumors. The pleomorphic ad- enoma is a benign, well-differentiated, well- circumscribed mass that grows slowly over the course of months to years. On histopathology, it is characterized by the presence of multiple cell types, classically epithelial cells in a chon- dromyxoid stroma.
43. A 45-year-old woman who was diagnosed with scleroderma five years ago presents to her physician with increasing difficulty swallow- ing. Which of the following abnormalities of esophageal muscle function is the most likely cause of these symptoms?
43. The correct answer is B. The upper third of the esophagus is made up of striated muscle (allows some voluntary control). The middle third of the esophagus is made up of both stri- ated and smooth muscle. The lower third of the esophagus is made up of smooth muscle (entirely involuntary). Patients with sclero- derma develop dysphagia (usually to solids) secondary to atrophy of smooth muscle of the lower two-thirds of the esophagus and incom- petence of the lower esophageal sphincter (LES). The wall of the esophagus becomes thin and atrophic and can have regions of fi- brosis.
9. An 18-year-old man with no significant medi- cal history presents to the clinic with pain in the right lower quadrant, mild diarrhea, and fever. This has happened twice within the past 12 months, but he has been asymptomatic between episodes. The patient denies recent travel or camping. Physical examination re- veals a perianal fistula. The gross appearance of the terminal ileum from a similar patient is shown in the image. Which of the following screening measures would be least useful in this patient?
9. The correct answer is D. The picture of in- termittent abdominal pain, fever, and diar- rhea should lead you to a diagnosis of irritable bowel disease. The presence of an anal fistula strongly suggests Crohn disease rather than ulcerative colitis, given that it causes transmu- ral inflammation. There are no renal disorders associated with Crohn disease, so blood urea nitrogen and creatinine would not be reason- able screening tests.
44. A 78-year-old man is brought to the hospital because of fever and acute onset of left lower quadrant abdominal pain. About a week ago, he was seen by his family physician for painless rectal bleeding. Laboratory tests show: RBC count: 5 million/mm³ Hematocrit: 36% Hemoglobin: 12 g/dL WBC count: 93,000/mm³ Mean corpuscular volume (MCV): 75 fL Which of the following is the most appropriate follow-up test after the patient is discharged?
44. The correct answer is B. Painless rectal bleed- ing in an elderly individual (especially with a history of constipation or poor fiber intake) suggests diverticulosis, a condition in which the mucosa and submucosa herniate through the muscular layer of the GI tract (frequently along the sigmoid colon), forming pockets called diverticula. This patient's lower left quadrant abdominal pain indicates that he is now suffering from acute diverticulitis, which is inflammation of one or more diverticula. The laboratory values show marked leukocy- tosis, which is actually a common finding in acute diverticulitis. More notably, the patient appears to have iron-deficient, microcytic ane- mia (low hemoglobin and low MCV) This could result from his past bleeding episodes; however, it could also be a sign of chronic oc- cult bleeding from an undiagnosed carcinoma. After the patient is stabilized and the acute diverticulitis has resolved, the patient should undergo colonoscopy to rule out malignancy. Colonoscopy is contraindicated during an acute episode of diverticulitis due to increased risk of bowel perforation.
45. A 26-year-old man presents to the clinic with bradykinesia, rigidity, and resting tremor. Se- rum aminotransferase levels are mildly el- evated. A liver biopsy is shown in the image. What is the chance that this patient's sister will have the same condition?
45. The correct answer is B. This is a presentation of Wilson disease. The patient has parkinso- nian symptoms due to the death of neurons in the basal ganglia (particularly in the putamen and globus pallidus). In addition, the liver bi- opsy shows evidence of cirrhosis (although this is a trichrome stain and not a copper stain, so the histopathology findings alone in this case are not specific). Wilson disease is an autoso- mal recessive disease in which there is a mu- tation in ATP7B, a gene in chromosome 13 that encodes for a copper-transporting ATPase. Copper accumulates in the liver, basal ganglia, bones, joints, kidneys, and Descemet mem- brane in the cornea (Kayser-Fleischer rings). Because Wilson disease follows an autosomal recessive pattern of inheritance, the patient's sister has a 25% chance of also having the dis- ease.
46. A woman comes to the physician because of profuse vomiting and watery, non-bloody diar- rhea that developed five hours after she had eaten tuna salad. She is diagnosed with food poisoning. Which of the following is the most likely cause of her symptoms?
46. The correct answer is C. The rapid onset of vomiting and diarrhea associated with Staphy- lococcus aureus food poisoning is not due to the bacterium itself but rather to ingestion of pre-formed enterotoxin. Mayonnaise and egg products are common sources. Answer A is incorrect. Bacillus cereus is a gram-positive, b-hemolytic rod that is often associated with food poisoning from reheated rice. Much like S aureus, it can produce a pre-formed toxin that can result in food poison- ing.
47. A newborn develops marked jaundice and kernicterus within weeks of birth. Blood tests reveal alanine aminotransferase = 16 U/L, as- partate aminotransferase = 14 U/L, and total bilirubin = 3.8 mg/dL. Urine tests are negative for bilirubin. Treatment attempts with pheno- barbital, plasmapheresis, and phototherapy are unsuccessful. The infant's condition dete- riorates over the next two months and he dies. What is the underlying cause of the infant's death?
47. The correct answer is B. The patient has Crigler-Najjar syndrome type 1. Patients with this condition lack uridine diphosphate glu- curonyl transferase, leading to an inability to conjugate bilirubin. This leads to increased unconjugated bilirubin, which causes jaun- dice, kernicterus, and bilirubin deposition in the brain. Crigler-Najjar syndrome type 1 pre- sents early in life and it is fatal. Type 2 of the syndrome is less severe and responds to pheno- barbital.
48. A 45-year-old man presents to the emergency department complaining of a high fever, mal- aise, and confusion since waking up earlier in the morning. He underwent abdominal sur- gery two weeks ago and was discharged two days postoperatively without complication. On examination, his temperature is 39.0°C (102.2°F), heart rate is 110/min, blood pres- sure is 80/50 mm Hg, and respiratory rate is 18/min. His abdomen is warm and erythema- tous, there is purulent discharge draining from the surgical incision site, and a rash is evident on his chest and abdomen. The patient re- ceives appropriate therapy. Days later, a blood culture reveals high levels of gram-positive bacteria, and molecular studies reveal high lev- els of interleukin (IL)-1, IL-6, and tumor ne- crosis factor-a. What cellular process initiated this patient's presentation?
48. The correct answer is E. The clinical presen- tation (fever, rash, hypotension) is character- istic of toxic shock syndrome (TSS) produced by Staphylococcus aureus. The S aureus TSST-1 superantigen simultaneously binds the b region of the T-cell receptor and the major histocompatibility complex class II of antigen-presenting cells, leading to the release of interferon-γ from T helper cells type 1 and IL-1, IL-6, and tumor necrosis factor-a from macrophages. Prolonged use of a single tam- pon is a common cause of TSS in women, but other causes in either sex include staphylococ- cal infection of surgical wounds, burns, and catheters.
49. A 40-year-old white man presents to the emer- gency department complaining of burning retrosternal chest pain after meals. The pain is relieved by antacids. The patient's ECG is normal, and x-ray of the chest is remarkable for an 8-cm hiatal hernia. This patient is at risk for developing which of the following types of cancer?
49. The correct answer is A. The quality and loca- tion of this patient's pain, combined with its al- leviation with medication and the radiographic findings, are suggestive of GERD. GERD increases the risk of developing esophageal mucosal metaplasia (squamous is replaced by columnar), called Barrett esophagus. In turn, patients with Barrett esophagus are at in- creased risk for developing adenocarcinoma of the distal esophagus. Barrett esophagus is di- agnosed by endoscopy, and must be followed with annual endoscopy and biopsy to monitor for adenocarcinoma.
5. A 25-year-old man presents to his physician with a complaint of "yellow eyes" for the past day. For the past five days, he has been ill with a low-grade fever, rhinorrhea, myalgias, and generalized malaise. The physical examination confirms scleral icterus, but is otherwise un- remarkable. Electrolytes and complete blood cell count are all within normal limits. Labora- tory tests show: Aspartate aminotransferase: 31 IU/L Alanine aminotransferase: 25 IU/L Alkaline phosphatase: 45 IU/L Total bilirubin: 3 mg/dL Lactate dehydrogenase: 40 IU/L Haptoglobin: 76 mg/dL (normal: 46-316 mg/ dL) His urinalysis demonstrates a normal bilirubin level. What is the most appropriate treatment for this patient's condition?
5. The correct answer is B. This patient has Gil- bert syndrome, the most common inherited disorder of bilirubin conjugation. It is due to a gene mutation that results in a decreased level of uridine diphosphate glucuronyl transferase (UDPGT). The disease is autosomal reces- sive; in the Western world, approximately 9% of people are homozygous for the mutation, and 30% are heterozygous (heterozygotes are asymptomatic). The disease commonly first manifests in young adults after an inciting event, such as a febrile illness, physical exer- tion, stress, or fasting. Laboratory tests will demonstrate unconjugated hyperbilirubin- emia, but will be otherwise normal. The dis- ease is benign and requires no treatment. This patient has a mild unconjugated hyperbilirubi- nemia and is otherwise healthy besides a mild flu-like illness, so Gilbert is the most likely di- agnosis.
50. A 50-year-old man presents to his physician because of a 6.8-kg (15-lb) weight loss over the past month, epigastric pain radiating to the back, and jaundice. He also complains of red- ness, swelling, and tenderness of his left lower extremity. Laboratory studies show an amylase level of 500 U/L, a lipase level of 300 U/L, and an alkaline phosphatase level of 500 U/L. Which tumor markers are most likely to be el- evated in this patient?
50. The correct answer is E. This patient has signs and symptoms characteristic of pancreatic ad- enocarcinoma. This malignancy often presents with jaundice, epigastric pain radiating to the back, and weight loss. The red, swollen, ten- der lower extremity indicates a possible deep vein thrombosis. Patients with pancreatic ad- enocarcinoma may present with migratory thrombophlebitis, which is called Trousseau sign. Laboratory studies show increased amy- lase, lipase, and alkaline phosphatase levels. Tumor markers such as CA 19-9 and carcino- embryonic antigen (CEA) are often elevated in pancreatic cancer. These markers are generally not sensitive or specific enough to be used for diagnosis, but they do have use in monitor- ing the course of the disease and response to therapy. CEA is also a marker for colorectal cancer. Pancreatic cancers are more common in patients with a history of smoking, DM, and chronic pancreatitis. Treatment for pancreatic adenocarcinoma is surgical removal, yet for most patients this is impossible, as the cancer has already metastasized prior to its discovery. If possible, pancreaticoduodenectomy or distal pancreatectomy is preferred to a total pancre- atectomy in order to preserve some of the pan- creatic function.
16. A 34-year-old man is brought to the emergency department after being involved in a high- speed collision with an oncoming car. He has multiple fractures and contusions. Results of fundoscopy are shown in the image. The pa- tient is stabilized and transferred to the inten- sive care unit. Two days later, there is evidence of gastrointestinal (GI) hemorrhage. What is the most likely mechanism of the GI bleeding?
6. A 34-year-old man is brought to the emergency department after being involved in a high- speed collision with an oncoming car. He has multiple fractures and contusions. Results of fundoscopy are shown in the image. The pa- tient is stabilized and transferred to the inten- sive care unit. Two days later, there is evidence of gastrointestinal (GI) hemorrhage. What is the most likely mechanism of the GI bleeding?
6. A 39-year-old white woman who suffers from polycythemia vera presents to the clinic com- plaining of severe and constant right upper quadrant pain over the past two days. Physical examination reveals an enlarged liver. What other finding would most likely be seen at pre- sentation?
6. The correct answer is A. This is an acute pre- sentation of Budd-Chiari syndrome, or throm- bosis of two or more hepatic veins. This condi- tion is associated with hypercoagulable states such as myeloproliferative disorders, inherited coagulation disorders, intra-abdominal can- cers, oral contraceptive use, and pregnancy. The increased intrahepatic pressure leading to ascites is present in 90% of patients with Budd- Chiari syndrome. The disease can also present in a subacute manner or in a chronic man- ner, and diagnosing this condition may then be more challenging because the classic triad of abdominal pain, hepatomegaly, and ascites may not be present.
7. A 29-year-old woman presents to her primary care physician complaining of "trouble eat- ing." She says she has had pain when swallow- ing both solids and liquids for the past nine months. She states that it has been difficult to maintain an appetite over this time and reports a weight loss of 2.3 kg (5 lb). Symptoms have remained constant since they appeared nine months ago. The patient does not exhibit tight- ening of the facial skin, claw-like hands, or any other systemic symptoms. Which of the follow- ing drug mechanisms of action is most likely to improve the patient's symptoms?
7. The correct answer is A. The patient's dys- phagia for both solids and liquids suggest a motility problem. Due to loss of the myen- teric (Auerbach's) plexus, the lower esophageal sphincter (LES) fails to relax. The majority of patients with achalasia have difficulty swallow- ing both solids and liquids for >6 months, as well as an increased risk of esophageal cancer. Esophageal manometry is the gold standard of diagnosing achalasia by documenting loss of coordinated peristalsis along the esophagus and abnormally high lower esophageal sphinc- ter tone. Upper endoscopy is also performed to rule out cancer. Surgical corrections include pneumatic dilation and esophageal myotomy. Medical approaches to treating achalasia in- clude using a calcium channel blocker, nitro- glycerin, or botulinum toxin. Calcium chan- nel blockers such as nifedipine decrease the availability of calcium to the myosin-actin complex, leading to smooth muscle relax- ation. Nitroglycerin works through a cGMP- mediated mechanism to dephosphorylate and inactivate myosin light chains. Botulinum toxin causes muscle paralysis by inhibiting the exocytosis of acetylcholine from presynpatic neurons.