Sherris Microbiology
A 5-year-old girl has a sore throat. She is febrile and has a scant exudate on one tonsillar pillar. The most sensitive way to detect whether this infection is due to group A streptococci is: A Throat culture B Streptococcal group A antigen detection C Streptococcal M protein antigen detection D Gram stain E ASO titer
ANSWER: A - Classically the most sensitive method to detect GAS. Answer analysis: A. CORRECT - Molecular methods (PCR) may be equally sensitive if available. B. Due to lower sensitivity must confirm negative results with culture. C. Not available D. Not specific E. Requires weeks to develop.
A child has had abdominal pain and diarrhea for 2 days. A stained preparation of the stool demonstrates polymorphonuclear leukocytes. Which of the following is least likely to produce these findings? A Vibrio cholerae B Shigella sonnei C Shigella dysenteriae D Salmonella serotype Typhimurium E Campylobacter jejuni
ANSWER: A - Cholera toxin causes only hypersecretion not inflammation. Answer analysis: A. CORRECT - V. cholerae would only be found in endemic parts of the world. B. Shigella species are the most potent cause of purulent stools. C. In addition to PMS S. dysenteriae may invade the bloodstream. D. Modest generator of PMNs evident only microscopically. E. PMNs are present early in the disease.
During a urinary tract infection, a 30-year-old woman developed hypotension, shock, and purpura. Gram-negative rods were discovered in the blood stream. The shock state is most due to the action of: A Lipopolysaccharide (LPS) lipid A B Lipopolysaccharide (LPS) side chains C Peptidoglycan D Cytoplasmic membrane E Superantigen toxin
ANSWER: A - Lipid A is the toxic component of LPS endotoxin Answer analysis: A. CORRECT - This is part of the gram-negative outer membrane. B. The side chains have other functions. C. Can produce symptoms if fragments circulate but not full-blown shock. D. Non-toxic E. Present in gram-positive staphylococci and streptococci.
An outbreak of diarrhea has spread through a Colorado day-care center caring for 3- to 5-year-old children. No food is served in the center. Which of the following organisms is most likely to be spread directly from child to child by the fecal-oral mechanism? A Shigella B Salmonella serotypes C Salmonella serotype Typhi D Enterotoxigenic E coli E Listeria
ANSWER: A - The low infecting dose of all Shigella species facilitates this kind of spread. Answer analysis: A. CORRECT - Shigella can also be transmitted in food. B. If food was served Salmonella would be a prime suspect. C. Typhoid in Colorado would require a rare carrier preparing food. D. High infecting dose. E. Spread in food not direct contact
A teenage boy has developed a tender, painful, lump in his axilla. The lesion eventually came to a "point" and drained purulent material. A Gram stain of the pus revealed WBCs and gram-positive cocci. Which of the following would be the most probable source of this infection? A Resident microbiota B Food C Insect bite D Swimming pool E Pet
ANSWER: A - This boil is the classic lesion of Staphylococcus aureus. Answer analysis: A. CORRECT - Shedding from the anterior nares to the skin is most common. B. Staphylococci can cause food poisoning but not this case. C. May provide an entry site but not the source. D. S. aureus not water-borne pathogen. E. Bite may provide entry site but not the source.
A 29-year-old male presents with a 2-day history of burning on urination and a thin, watery urethral discharge. He had unprotected sex with a new female partner four weeks ago. A Gram stain of the discharge reveals 50% polymorphonuclear (PMN) and 50% mononuclear leukocytes. No microorganisms were visible and a culture for gonococci was negative. Transmission to another sexual partner would be by acquisition of: A Elementary body B Reticulate body C Outer membrane lipopolysaccharide D Peptidoglycan E Invasin
ANSWER: A - This is Chlamydia trachomatis genital infection. Answer analysis: A. CORRECT - The elementary body is the infectious form of C. trachomatis. B. This is the reproductive form multiplying inside human cells. C. Present but not involved in staining D. Absent in C. trachomatis E. Unknown in C. trachomatis
A 20 year-old African woman has had progressively worsening inflammation of both eyes since childhood. It primarily involves the conjunctiva which are now covered with follicular nodules. Which of the following best describes the features of this pathogen: A Gram-negative, oxidase positive cocci. B Gram-positive cocci in clusters. C Obligate intracellular cocci, biovar A D Obligate intracellular cocci, biovar L E Spirochete, darkfield positive
ANSWER: A - This is advanced trachoma caused by Chlamydia trachomatis. Answer analysis: A. Gonococci cause acute conjunctivitis primarily in newborns. B. Staphylococcus aureus is another cause of acute conjunctivitis. C. CORRECT - Biovars A, B, Ba, and C cause trachoma. D. Biovars L1, L2, and L3 cause lymphogranuloma venereum. E. Not causes of conjunctivitis.
A diarrhea outbreak has taken place among toddlers in a Chicago day care center. The children come from a wide area and no food is allowed in the center. The stools are small volume and microscopically show WBCs and RBCs. A stool culture has detected a gram-negative rod known to be an enteric pathogen. In the host the primary feature associated with this pathogen is: A Enterocyte to enterocyte spread B Bloodstream invasion C Ruffle-like structures on M cells D Stimulation of secretion E Biofilm formation
ANSWER: A - This is shigellosis spreading by direct contact due to low infecting dose. Answer analysis: A. CORRECT - The radial nature of this spread produces multiple ulcers. B. Infrequent except with Shigella dysenteriae Type 1 (unlikely in Chicago). C. Typical of Salmonella. D. Vibrio cholerae and enterotoxigenic E. coli. E. Not involved
A number of residents of a migratory worker camp in Arizona have developed fever and night sweats that seem to come and go each day. For some it has been going on more than a month. There are no signs which point to any organ system, but most had eaten from a large supply of cheese brought by one of them from Mexico. The best way to establish a diagnosis in these workers is: A Blood culture B Gram stain C Sputum culture D CSF culture E Serology
ANSWER: A - This is undulant fever caused by Brucella abortus. Answer analysis: A. CORRECT - The fevers are caused by circulating bacteria. B. There is no local lesion to stain. C. No pneumonia, no sputum. D. CNS invasion uncommon E. Available but less sensitive and specific than blood culture.
Which of the following biologic substances most commonly functions as the human receptor for bacterial adherence? A Ribosome B Fibronectin C Cholesterol D Polysaccharide E Pili
ANSWER: B - A broad spectrum binder and widely present on human cell surfaces. Answer analysis: A. Inside the cell so not available. B. CORRECT - On the surface of endothelial cells throughout the body. C. Human lipid but not usually on cellsurfaces. D. Present on the bacterial not human cell. E. Common binder but from the bacterial side.
An elderly man with an enlarged prostate has frequent and painful urination. Suddenly he develops fever and chills. Examination reveals hypotension (blood pressure 55/10 mm Hg) and a blood culture is positive for Klebsiella pneumoniae. The fever, chills, and hypotension likely derive from the bacterial: A Alpha toxin B Outer membrane C Polysaccharide capsule D Endoplasmic reticulum E Polypeptide capsule
ANSWER: B - All these findings are produced by LPS endotoxin. Answer analysis: A. A mediator of local not systemic injury. B. LPS is located in the gram-negative outer membrane. C. K. pneumoniae may have a capsule but it would not be the direct cause of these symptoms. D. Not found in bacteria. E. Found only in a few gram-positive bacteria.
A young woman has been identified as a sexual partner of a man recently diagnosed with gonococcal urethritis. She is in good health with no genital pain or discharge. Of the methods listed below which is the best way to determine if she has gonorrhea: A Pilin serology B Opa serology C Gram stain D Vaginal culture E Cervical culture
ANSWER: E - Clinically proven to give the highest yield Answer analysis: A. Not available and unlikely candidate due to antigenic variation B. Same as above C. Over 90% sensitive in males but less than 50% in females. D. 70% sensitive E. The standard. Molecular methods (if available) are now equal.
A transient man is seen in the emergency room for fever, chills, and a productive cough. The episode began suddenly with a severe shaking chill. A Gram stain of his sputum shows gram-positive cocci in pairs and his chest X-ray shows consolidation of the right middle lobe. Which feature of the bacterium was most important in the initiation of this infection? A Pili adhering to tracheal mucosa B Polysaccharide interference with complement C Catalase generating superoxide ions D Superantigen generation of cytokines E Cell injury by pore-forming toxi
ANSWER: B - Its capsule interferes with C3b deposition on the pneumococcal surface. Answer analysis: A. Pili not involved. B. CORRECT - The result is blocking ingestion by professional phagocytes. C. Pneumococci do not produce catalase. D. No superantigens produced E. No pore-forming toxins produced.
The characteristic of the organism which causes tuberculosis which best distinguishes it from other genera is: A Thick peptidoglycan B High cell wall lipid content C Impermeable outer membrane D Injection secretion system E Lancefield carbohydrate
ANSWER: B - Mycolic acids are prominent in the cell walls of all mycobacteria. Answer analysis: A. Similar to other gram-positive bacteria. B. CORRECT - This is responsible for the acid-fast staining characteristic. C. No outer membrane D. No known secretion system E. Only found in group A streptococci
The incidence and prevalence of syphilis is increasing worldwide. Which of the following potentially contributing factors is LEAST RESPONSIBLE for this rise? A Intravenous drug usage B Antimicrobial resistance C HIV prevalence D Patient contact tracing E Lack of effective diagnostic tools
ANSWER: B - Penicillin is still universally effective. Answer analysis: A. Major cause of non-sexual transmission B. CORRECT - For those hypersensitive to penicillin there are effective alternatives. C. The deadly combination of HIV and syphilis has enhanced the spread of both. D. The resources are not available in many countries E. Even without culture serologic tests are very effective
A 70-year-old woman has collapsed following weeks of nausea and upper abdominal paid. In the emergency room she is discovered to have a perforated gastric ulcer. Which of the following features is significant in both the pathogenesis and diagnosis of this disease? A Coagulase B Urease C CagA protein D Superantigen exotoxin E Alginate
ANSWER: B - This is Helicobacter pylori disease. The potent urease assists H. pylori survival in the acid stomach. Answer analysis: A. Staphylococcus aureus diagnosis. B. CORRECT - Urease activity in gastric secretions can be detected in less than an hour. C. Important in pathogenesis but not diagnosis D. Staph or strep toxic shock syndrome. E. Pseudomonas biofilm
A few days after completing a 3-day hike in Arizona's White Mountains a young man developed a high fever followed by a petechial rash spreading from the extremities, including palms and soles, to the trunk. This infection was most likely acquired from: A Deer B Ticks C Water D Air ducts E Rat fleas
ANSWER: B - This is Rocky Mountain Spotted Fever which is transmitted by the bite of a number of tick species, Answer analysis: A. Animals not involved. B. CORRECT - The largest number of cases occur in the wooded areas of eastern and midwestern states. C. No aerosol spread. D. Not found in the woods. E. The vector for endemic (murine) typhus.
A few days after returning from vacation in a developing country, multiple members of a family developed diarrhea. Routine stool cultures were negative, but a molecular panel detected an E. coli strain associated with intestinal disease. If the stool shows no blood or inflammatory cells this strain most likely produces: A Shiga toxin (Stx) B Labile toxin (LT) C Attaching and effacing (A/E) lesions D Invasion, ulcers E A and B F A and C
ANSWER: B - This is enterotoxigenic E. coli which stimulates only hypersecretion with no mucosal injury. Answer analysis: A. Stool would show blood B. CORRECT - Stable toxin (ST) would likely be produced as well C. Mostly in newborns D. Stool would show blood and WBCs E. Not seen together F. A/E lesions and bloody stools
A 30-year-old man presents with fever, headache, and a decline in mental status. He was previously healthy and had received the standard immunizations in school. A lumbar puncture reveals more than 100 white blood cells per milliliter of cerebrospinal fluid (CSF). If a CSF Gram stain reveals gram-negative diplococci, which of the following actions would best prevent spread of the infection to others in the family? A Penicillin chemoprophylaxis B Rifampin chemoprophylaxis C Polysaccharide vaccine D Wearing masks E Handwashing
ANSWER: B - This is meningococcal meningitis. Rifampin should be given to all family members and close contacts. Answer analysis: A. Penicillin penetration into uninflamed nasopharynx is poor. B. CORRECT - Outbreaks in schools etc. requires wider prophylaxis. C. Done for unimmunized but will not act fast enough for current exposure. D. Indicated in places like hospitals. E. Always a good idea but ancillary here.
An 8-year-old boy has been listless and irritable for a week. The mother says he had a sore throat 3 weeks ago but did not see a physician because the family lacks healthcare coverage and "it wasn't that bad." Examination reveals arthritis in two joints and a heart murmur. His antistreptolysin O (ASO) titer is elevated. His cardiac findings are most likely due to antibody stimulated by: A Streptococcal superantigen toxin B M protein C Streptolysin O D Lipoteichoic acid E Fibronectin
ANSWER: B - This is rheumatic fever, an autoimmune reaction to Group A streptococci. Answer analysis: A. Possibly involved in antecedent pharyngitis if signs of scarlet fever. B. CORRECT - M protein binds to heart sarcolemma stimulating antibody C. Responsible for ASO antibodies but not injury. D. Structural part of GAS cell wall. E. Glycoprotein coating many human membranes.
A sexually active, 30-year-old male patient has a history of genital ulcer which healed several weeks ago. He now presents with a maculopapular rash over his entire body, extending to the palms, soles, and face. Examination of one of the skin lesions by what method would be most likely to demonstrate the causative agent of this infection? A Gram stain B Darkfield microscopy C Modified acid-fast stain D Acid-fast stain E Culture
ANSWER: B - This is secondary syphilis caused by Treponema pallidum. Answer analysis: A. T. pallidum does not stain by gram. B. CORRECT - Spirochetes can be seen if the specimen contains blood. C. T. pallidum not acid-fast. D. Same as above. E. T. pallidum has not been grown in culture.
Only one form of typhus occurs in human epidemics. In this disease the vector is: A Tick B Mite C Mite larvae (chiggers) D Flea E Body louse
ANSWER: E - In epidemic typhus crowding and extremely poor sanitary conditions allow body lice to pass from person to person. Answer analysis: A. Many diseases but one human at a time. B. Rickettsialpox C. Scrub typhus. D. Murine (endemic) typhus E. CORRECT - The body louse acquires R. prowazekii from the blood of a patient.
A 3-month-old infant was admitted to the hospital with a 10-day history of repetitive coughing and choking spells. His white blood cell count was 30 000/mm3 (normal <10 000/mm3) with 70% lymphocytes. The child's chest radiograph was clear. Which of the following methods is the most sensitive method for making an etiologic diagnosis: A Sputum culture on blood agar B Nasopharyngeal culture on special medium C "Cough plate" culture on special medium D Sputum Gram stain E Sputum acid-fast stain
ANSWER: B - This is whooping cough caused by Bordetella pertussis. Answer analysis: A. B. pertussis will not grow on blood agar. B. CORRECT - A special charcoal agar made selective with antibiotics is required. C. Less sensitive specimen collection than NP swab. D. Non-specific E. B. pertussis is not acid-fast.
The functional bacterial structure most likely to be acquired by one bacterial cell from another using the conjugation mechanism is: A Circular chromosome B Bacteriophage C Plasmid D DNA fragment E Transposon
ANSWER: C - Able to traverse the conjugation bridge and either reform in the cytoplasm or integrate into the recipient cell chromosome. Answer analysis: A. Too large B. Enters by phage injection C. CORRECT - Large enough to include multiple genes. D. Not functional E. May be present in the plasmid
All of the following clinical syndromes are known to be produced by Haemophilus influenzae EXCEPT: A Meningitis B Epiglottitis C Chancroid D Otitis media E Bronchitis
ANSWER: C - Chancroid is caused by Haemophilus ducreyi. Answer analysis: A. Only encapsulated strains particularly type b. B. Same as above. C. CORRECT - A different species found primarily in tropical areas. D. Both encapsulated and non-encapsulated strains. E. Mostly non-encapsulated strains
A few days after birth, a newborn developed an umbilical infection from which gram-positive cocci in clusters were isolated. The next day he appeared "sunburned" and the superficial layers of his skin peeled away. Except for elevated WBC count routine, hematologic and chemistry tests were normal. The cutaneous findings in this case are most likely due to: A Endotoxin B Staphylococcal superantigen toxin C Exfoliatin D Peptidoglycan E Coagulase
ANSWER: C - Exfoliatin is the mediator of staphylococcal scalded skin syndrome. Answer analysis: A. No endotoxin in gram positives. B. StaphSAg disease would include shock. C. CORRECT - The delicate skin of newborns allows exfoliatin to be absorbed and circulate. D. Not toxic E. Not toxic
Congenital syphilis is best prevented by: A Culture in early pregnancy B Culture in late pregnancy C Serology in early pregnancy D Serology in late pregnancy E Darkfield exam at birth
ANSWER: C - Fetus is not susceptible until after the 4th month of pregnancy. Answer analysis: A. There is no culture for Treponema pallidum B. Same as above C. CORRECT - treatment will prevent transmission to fetus. D. Fetus may already be infected. Of course, treat anyway. E. Too late.
A bacterial strain has acquired a set of genes by transduction. The process entered the lysogenic cycle and the bacterial cells have gone through a cycle of reproduction. In the daughter cells these genes will most likely be found in: A Bacteriophage B Plasmid C Chromosome D Transposon E Cytoplasmic membrane
ANSWER: C - In transduction the chromosome is the most likely destination of the injected DNA or RNA. Answer analysis: A. The bacteriophage is the source not the destination of the nucleic acids. B. Less likely that the chromosome. C. CORRECT - Integrated genes may be replicated for multiple generations. D. Not linked to transduction E. Same as above
If an E coli is introduced into the urinary bladder by mechanical disruption of the perineal flora, which of the following characteristics would give it the best chance to produce pyelonephritis? A Alpha hemolysin B CFA pili C P (gal-gal) pili D Type 1 pili E LPS endotoxin
ANSWER: C - P pili concentrated in the upper urinary tract near the kidneys. Answer analysis: A. Pore-forming toxin B. Bind to intestinal enterocytes. C. CORRECT - Pyelonephritis also increases the probability of bacteremia. D. Common pili attaching in and outside the bladder. E. Injury mediator in the bladder and bloodstream.
A 12-year-old girl woke up saying it hurt to swallow. Her mother took her to a physician who said it looked like a viral pharyngitis and performed a rapid strep antigen test which was negative. Three weeks later she became listless and irritable. Physical examination revealed a febrile girl with arthritis in two joints and a heart murmur. Her antistreptolysin O (ASO) titer was elevated. Her cardiac findings are most likely due to antibody directed against: A Superantigen toxin B Streptolysin O C Sarcolemmal membranes D Lipoteichoic acid E Adhesive pili
ANSWER: C - This antibody is stimulated by group A streptococcal M protein. Answer analysis: A. These toxins bypass the antibody step B. The antigen responsible for the ASO titer. C. CORRECT - A classical molecular mimicry (Type II) hypersensitivity reaction. D. Involved in adherence. E. Same as above
A college student has felt "off" for a week. At the campus clinic she is found to be febrile and radiographs reveal a surprising degree of lung lower lobe infiltrates. She is producing no sputum. A batch of serologic tests are negative for viruses but positive for cold agglutinins. The most unique feature of the probable etiologic agent is: A Polysaccharide capsule B Lack of outer membrane C Lack of cell wall D Adherent pili E Pore-forming toxin
ANSWER: C - This is "walking pneumonia" caused by Mycoplasma pneumoniae. Answer analysis: A. Does not form capsule B. True but not the definitive feature. C. CORRECT - With no cell wall does not stain by Gram. D. Present but not unique E. Not involved and not unique
A middle-aged farmer has developed fever, dry cough, headache and hepatosplenomegaly. Radiographs reveal interstitial pneumonia and abnormal liver function tests. Cultures are negative but serologic testing reveals antibodies directed against a gram-negative rod now known to pathogenically resemble Legionella pneumophila. What is most likely to be the primary reservoir of this pathogen? A Stream water B Cattle and sheep feces C Mammalian placental tissue D Ticks E Amoebas
ANSWER: C - This is Q fever caused by Coxiella burnetii. Answer analysis: A. Water not involved as it is with Legionella B. Cattle yes, but not feces C. CORRECT - Dried parturition products create aerosols which are inhaled, sometimes at great distance. D. Not required E. Not involved
Five post office workers have all come down with a similar respiratory illness characterized by low-grade fever, chills, cough, dyspnea on exertion, and generalized malaise. A chest radiograph taken on one of them shows mediastinal edema and a sputum Gram stain shows WBCs and large gram-positive rods. A blood culture also shows gram-positive rods. This infection was most likely acquired by: A Inhalation of vegetative bacteria B Inhalation of conidia C Inhalation of spores D Traumatic inoculation of vegetative bacteria E Traumatic inoculation of spores
ANSWER: C - This is a case of anthrax acquired by inhalation of spores planted in envelopes. Answer analysis: A. Vegetative bacteria would not survive long enough. B. Conidia are fungal structures similar to spores C. CORRECT - This is bioterrorism D. See A E. This would lead to localized lesion at that site
A young girl in the former Soviet republic of Georgia has a severe sore throat and multiple white plaques in the back of her throat. She is acutely ill and has a heart murmur. She has had only one set of her childhood immunizations because the supplies in the village where she lives had run out. The manifestations in her throat and heart are due to a toxin which: A Stimulates adenylate cyclase B Inserts into sarcolemma membranes C Inhibits protein synthesis D Inhibits acetylcholine release E Stimulates cytokine release
ANSWER: C - This is a case of diphtheria Answer analysis: A. A mechanism for toxins that usually stimulate secretion like diarrhea B. M protein injury mechanism in rheumatic fever C. CORRECT - Diphtheria toxin (DT), leads to cell death D. Cause of paralysis in botulism E. A range of activities (good and bad) depending on the cells cytokines reach
A newborn was delivered at home without medical assistance. The umbilical cord was cut with kitchen shears. The baby initially did fine but in the third week of life began to have involuntary muscle contractions. The infant now has generalized muscle contractions and difficulty breathing. The abnormal muscle spasms are due to a toxin which: A Stimulates neuromuscular synapses B Stimulates neurotransmission in the spinal cord C Blocks postsynaptic inhibition in the spinal cord D Blocks acetylcholine release in the spinal cord E Blocks acetylcholine release at neuromuscular junctions
ANSWER: C - This is a case of tetanus from spores on the non-sterile shears. Answer analysis: A. It is a lack of regulation rather than direct stimulation B. Same as above C. CORRECT - This results in unopposed (unregulated) normal impulses D. The effect of this would be paralysis as in botulism E. Similar to above
In the pre-immunization era most cases of meningococcal meningitis occured between the ages of 6 months and 5 years (peak 18 months). The best explanation for this age distribution is: A This is the age when exposure is most likely B The T-cell dependent immune response is poorly developed at this age C Antimeningococcal antibody is less likely to be present at this age D Maternal antibody persists through this period E Maternal antibody is not protective
ANSWER: C - This is due to the combination of transplacental and naturally acquired immunity. Answer analysis: A. Exposure is highly variable B. This is true for the first year of life C. CORRECT - This means most develop immunity without disease. D. Transplacental IgG disappears throughout the first year. E. The absence of disease is the evidence it is protective.
Multiple newborns have developed diarrhea while still in the nursery. Routine stool cultures were negative, but a molecular panel detected an E. coli strain associated with intestinal disease. If the stool shows no blood but the mucosa show pedestal-like extensions this strain most likely produces: A Shiga toxin (Stx) B Labile toxin (LT) C Attaching and effacing (A/E) lesions D Invasion, ulcers E A and B F A and C
ANSWER: C - This is enteropathogenic E. coli produced by the A/E lesions and other proteins secreted into the enterocyte cytoplasm. Answer analysis: A. Combined with A/E lesions in enterohemorrhagic syndrome. B. Enterotoxigenic E. coli. C. CORRECT - In developing countries this syndrome is seen through the first year of life. D. Stool would show blood and WBCs E. Not seen together
A young woman has developed fever and hypotension three days into her menstrual cycle. Laboratory findings include leukocytosis and an elevated blood urea nitrogen. Blood cultures were negative, but a vaginal culture grew gram-positive cocci which were catalase and coagulase positive. The systemic findings are most likely due to production of: A Endotoxin B A/B toxin C Superantigen toxin D Pore forming toxin E Peptidoglycan
ANSWER: C - This is staphylococcal toxic shock syndrome. Answer analysis: A. Only present in gram-negative bacteria B. There are many of this but not this case. C. CORRECT - The StaphSAg TSST-1 was absorbed across the vaginal mucosa to the blood stream causing massive cytokine release. D. S. aureus a-toxin is pore-forming but is not causing the findings in this case. E. Major structural component of cell wall.
A 6-month-old infant presents with fever, hoarseness, and difficult breathing. Examination reveals a red, swollen, epiglottis. The laboratory reports that a blood culture is growing gram-negative coccobacilli. Immunity to infection with this organism is provided by antibodies directed against: A Cytotoxic T cells B M protein C Polyribitol phosphate D Surface pili E Outer membrane proteins
ANSWER: C - This is the dominant component of the Haemophilus influenzae type b polysaccharide capsule. Answer analysis: A. Not an immunopathologic process. B. Found in group A streptococci. C. Because Hib is overwhelmingly dominant in disease the vaccine is 99% effective. D. Present but not distinctive for invasive disease. E. Same as above.
Multiple members of the same family began to experience fever, chills, and night sweats two weeks after returning from a trip to rural Mexico. The fevers typically faded but came back night after night. While on the trip all had consumed unpasteurized goat cheese made on a friend's farm. A coccobacillary gram-negative rod was isolated from blood cultures after prolonged incubation. The primary reproductive site of this pathogen is: A Small intestine B Large intestine C Reticuloendothelial system D Cardiac valves E Lung
ANSWER: C - This is the typical "undulant fever" of Brucella abortus caused by periotic seeding from the reticuloendothelial system into the blood. Answer analysis: A. Available but less specific B. Useless. These are classic gram-negative rods. C. CORRECT - May take 4-5 days or more to grow in the laboratory. D. Enter by the GI tract but infect macrophages and the RES E. Urinary tract not involved.
A 25-year-old student has developed diarrhea with 8 to 10 stools a day. He was healthy 2 days earlier and has no known immune deficits. If the infection developed while traveling in a developing country and the stool contains neither red or white blood cells, the diarrhea is most likely due to: A Shiga toxin (Stx) B A protein synthesis inhibiting toxin C An ADP-ribosylating toxin D A pore-forming toxin E An invading bacterium
ANSWER: C - Travelers' diarrhea due to enterotoxigenic E. coli. Answer analysis: A. This produces bloody stools. B. These would produce blood and/or WBCs C. CORRECT - The toxins would be labile (LT) or stable (ST). D. Uncommon but would cause inflammation at least E. Like Shigella with both blood and pus.
Plague continues to exist in many parts of the world. Select the combination from the list that acts as the reservoir favoring this persistence? A Fleas and deer B Ticks and wild rodents C Fleas and wild rodents D Mosquitoes and urban rats E Fleas and urban rats
ANSWER: C - Wild rodents are the most important reservoir, fleas are the vector. Answer analysis: A. Deer for Lyme disease, not plague. B. Ticks for other diseases, not plague C. CORRECT - Urban rats in urban plague D. Mosquitoes not involved. E. Urban plague now too rare to act as reservoir
DID NOT LEARN For which of the following diseases are human red blood cells the primary target of the pathogen? A Rocky Mountain spotted fever B Typhus C Cat-scratch disease D Scrub typhus E Murine typhus
ANSWER: C -Bartonella henselae is the pathogen and RBCs the target. Answer analysis: A. Vascular endothelium B. Vascular endothelium C. CORRECT - Transferred cat to cat by fleas but whether the cat to human mechanism is more than a scratch is unclear. D. Mononuclear cells E. Vascular endothelium
A woman has been in the hospital for 3 weeks due to complications following surgery for colon cancer. Five days into a course of ceftriaxone for suspected pneumonia she developed diarrhea. Colonoscopy revealed multiple plaques on the mucosa which are composed of fibrin and WBS. Stool examinations for Salmonella, Shigella, Campylobacter, and amoebas are negative. The diarrhea and plaques are most likely produced by: A Endotoxin from clostridial spores B Exotoxin from clostridial spores C Exotoxin from clostridial cells D Labile Toxin from E coli cells E Endotoxin from E coli cells
ANSWER: C -Clostridium difficile produces potent exotoxins when able to multiply freely in the intestine. Answer analysis: A. Neither clostridial cells or spores contain endotoxin B. Spores are inert producing no metabolic products C. CORRECT - The ceftriaxone gave C. difficile an "edge" by inhibiting competing flora. D. LT causes a watery diarrhea with no enteric cell injury. E. Endotoxin does not cause diarrhea.
A 12-year old girl with cystic fibrosis has developed a cough and pulmonary infiltrates. A sputum culture reveals gram-negative rods which are oxidase positive. The most important pathogenic feature facilitating persistence of this infection is: A Pore-forming toxin B Adhesive pili C Biofilm formation D Polysaccharide capsule E Spore formation
ANSWER: C -Pseudomonas aeruginosa forms and alginate biofilm in this unique environment. Answer analysis: A. A potent toxin-former but not of this type B. The biofilm itself is the major colonizing factor C. CORRECT - Mutations in regulatory genes lead to overproduction of the alginate. D. Like a capsule but alginate chemically E. P. aeruginosa does not form spores.
Two isolates of Neisseria gonorrhoeae have been obtained from a woman with disseminated gonococcal infection (DGI). One is from the cervix, the other from the blood. Detailed studies of these isolates show that antibody directed against the pili of the cervical isolate neutralize its binding to epithelial cells, but the same antibodies have no effect on the blood isolate. The most likely explanation for these observations is: A She is infected with two strains from different partners B The cervical isolate has a plasmid which was lost in the blood C A translational frame shift has shut off the pili of the blood isolate D Recombination between pilin genes has altered the pili E The cervical isolate has a transposon lacking in the blood isolate
ANSWER: D - By this mechanism a second antigenic type has been generated by genetic interaction between unlinked genes in the same chromosome. Answer analysis: A. Theoretically possible but unlikely in this time frame. B. This might explain the loss of a variant but not the emergence of a new type. C. This mechanism has been observed for antigenic variation of outer membrane proteins not pili. D. CORRECT - This has been observed during the course of a single infection. E. This does not explain the emergence of a new antigenic type.
An elderly woman recovering from surgery had been in the hospital receiving intravenous fluids for 6 days. On the fifth hospital day she developed a low-grade fever. Physical examination and radiographs revealed no obvious source but 3 of 3 blood cultures taken were positive for gram-positive cocci which were catalase positive and coagulase negative. When her IV line was removed, the fever went away. What virulence feature of the organism facilitated this episode? A Pili adherent to epithelial cells B Pili adherent to plastic C Polysaccharide adherent to epithelial cells D Polysaccharide adherent to plastic E Pore-forming toxin
ANSWER: D - Coagulase negative staphylococcal biofilm adhere to catheters and many other implanted medical devices. Produced only by some CoNS. Answer analysis: A. Pili not involved B. Same as above C. Possible but not the case here. D. CORRECT - Plastic polymers in artificial heart valves are particularly important. E. No toxin required
A major difference between the structure of the gram-positive and gram-negative bacterial cell wall is that the gram-negative wall contains: A Peptidoglycan B Pili C Flagella D Outer membrane E Capsule
ANSWER: D - Only in gram negative bacteria. Answer analysis: A. Present in virtually all bacteria. B. Variable. C. Variable. D. CORRECT - Outer membrane LPS endotoxin only in gram negatives E. Variable
A 40-year-old man has dysuria and copious amounts of pus coming from the urethra. He has been with multiple sexual partners in the past 2 months. A Gram stain of the pus shows many gram-negative diplococci both in and outside neutrophils. He is also discovered to have a positive fluorescent treponemal antibody (FTA-ABS) test but a negative nontreponemal (RPR) test. What best describes his disease(s) state? A Gonorrhea (active) B Syphilis (active) C Gonorrhea (active) and syphilis (active) D Gonorrhea (active) and syphilis (previous) E Gonorrhea (previous) and syphilis (previous)
ANSWER: D - The Gram stain is diagnostic for gonorrhea and the negative RPR rules out active syphilis. Answer analysis: A. True but what about syphilis serologies? B. Negative RPR indicates not active. C. Same as above. D. CORRECT - The FTA-ABS remains positive for decades, even after successful treatment. E. Gonorrhea is active.
A college has adopted an aggressive immunization policy which includes use of the newest Hib, meningococcal, and pneumococcal vaccines. Despite this, an outbreak of meningitis has developed on campus with clear evidence of transmission between roommates. Which of the following would have the greatest potential to produce an outbreak under these circumstances? A Haemophilus influenzae, type b B Haemophilus influenzae, type a C Neisseria meningitidis, group A D Neisseria meningitidis, group B E Streptococcus pneumoniae, type 24
ANSWER: D - The group B meningococcus is the only one of these for which an effective vaccine is not available. Answer analysis: A. Even before development of the Hib vaccine disease was uncommon after age 7. B. Uncommon before and after new vaccine developments. C. Rare in USA except in pandemics separated by decades. D. CORRECT - There are now 2 candidate vaccines in trial. E. This and other serotypes occur at all ages but not in epidemics.
A young man has returned from a month in Africa. For the past 2 weeks he has had a slowly increasing fever and now night sweats. He recalls a few loose stools but has no diarrhea now. In order to rule out typhoid fever the best diagnostic test would be: A Serologic test B Stool culture C Urine culture D Blood culture E Rapid plasma reagin test
ANSWER: D - The typhoid symptoms at this time are due to S. Typhi bacteremia. Answer analysis: A. Less sensitive and specific B. Perhaps a few weeks ago but now less sensitive than blood. C. Sometimes positive secondary to bacteremia D. CORRECT - >90% positive at this stage of typhoid fever. E. A test for syphilis.
A purified polysaccharide vaccine was successful in preventing invasive meningococcal disease in military populations but not in children under 2 years of age. The most probable explanation for this involves: A Maternal IgG transfer at birth B CD8+ T-cell function C Bone marrow stem cells D Maturation of T-cell dependent responses E Maturation of T-cell independent responses
ANSWER: D - These are most important and for the meningococcus develop slowly (>2 years). Answer analysis: A. Effective but decline in the first year of life. B. Important for viral defense. C. Involved only in general way. D. CORRECT - These are stimulated by protein-conjugate vaccines. E. Weak and decline rapidly.
A 35-year-old Connecticut woman has suffered recurrent large joint arthritis for 5 years. Which of the following would give the best support for a diagnosis of chronic Lyme disease as the cause? A Joint darkfield exam B Positive enzyme immunoassay C History of erythema nodosum D History of erythema migrans E History of hiking in wooded areas
ANSWER: D - This is an unusual syndrome which establishes the primary infection. Answer analysis: A. Rarely positive in any stage of Lyme disease B. Sensitive but specifically also requires confirmatory Western blot C. Seen in coccidioidomycosis. D. CORRECT - Particularly important for those outside the usual endemic area. E. Living in this endemic area is good enough
A few days following a large summer picnic multiple people developed mild abdominal pain and a diarrhea with small stool volumes. Routine stool cultures were negative, but a molecular panel detected an E. coli strain associated with intestinal disease. If the stool microscopy demonstrated RBCs and WBCs this strain most likely produces: A Shiga toxin (Stx) B Labile toxin (LT) C Attaching and effacing (A/E) lesions D Invasion, ulcers E A and B F A and C
ANSWER: D - This is enteroinvasive E. coli a mild version of shigellosis. Answer analysis: A. Combined with A/E lesions in enterohemorrhagic syndrome B. Enterotoxigenic, no RBCs or WBCs C. Enteropathogenic E. coli D. CORRECT - Humans only reservoir E. Not seen together F. The combination in enterohemorrhagic E. coli disease.
The owner of a large pet store has developed fever, headache, and a dry hacking cough. Radiographs reveal a bilateral interstitial pneumonia. Gram and acid-fast stains are negative as are bacterial and fungal cultures. A nucleic acid amplification (PCR) panel for respiratory viruses is also negative. Which of the following is the most likely source of this infection? A Snakes B Salamanders C Puppies D Birds E Rodents
ANSWER: D - This is most likely human psittacosis inhaled from the aerosolized droppings of exotic birds. Answer analysis: A. Would require direct contact B. Same as above C. Pulmonary unlikely D. CORRECT - The agent Chlamydophila psittaci is similar to the chlamydia. E. Usually requires a vector
A 56-year-old man has had a cough with hemoptysis for six weeks and has lost 25 pounds. His chest X-ray reveals a right upper lobe cavity. His sputum shows multiple slender acid-fast bacilli. The primary mechanism of injury to his lung is: A Lipopolysaccharide endotoxin B Protein exotoxin C Pore-forming toxin D Delayed type hypersensitivity E Immune complex deposition
ANSWER: D - This is pulmonary tuberculosis caused by Mycobacterium tuberculosis, probably reactivation from an infection acquired years ago. Answer analysis: A. MTB does not have LPS endotoxin. B. MTB has no known exotoxins C. Same as above D. Failure to control MTB growth leads to destructive DTS response. E. Not seen in tuberculosis.
Plague occurs in two major epidemiologic cycles, sylvatic and urban. In the historic pandemics mortality rates were much higher in the urban outbreaks. The most probable reason for this is: A Rat transmission more efficient B Urban population more susceptible C Antibiotics less effective D Pneumonic plague is more frequent E Relapses more frequent
ANSWER: D - Urban setting enhances human-to-human transmission of pneumonic plague which has a 100% mortality untreated. Answer analysis: A. Even if true would increase the number of cases not the mortality rate. B. Any effect dwarfed by higher pneumonic fatality rate. C. Antibiotics save some if given early D. CORRECT - The Black Death also has a shorter incubation period. E. Pneumonic plague victims do not have time to relapse.
A 9-year-old boy returned home after attending summer camp in Rhode Island. Upon his return he had complaints of recurrent fever, muscle aches, severe headaches, and fatigue. The patient also had an annular (ring-like) rash on his left arm and later developed a facial palsy. In order to consider a diagnosis of Lyme disease what additional history would be most helpful? A Food consumption B Swimming in lakes or streams C Sexual contact D Hiking locales E Illness of friends
ANSWER: D -Borrelia burgdorferi is transmitted by ticks following a complex life cycle. Answer analysis: A. No food only a tick bite, B. Ticks not found here. C. No human source. D. CORRECT - The life cycle involves mice and deer in addition to ticks, all found in wooded areas. E. Same as C above.
In the investigation of a young man's vague inflammatory complains cells with large cytoplasmic inclusions were discovered in the cerebrospinal fluid. He recalled tick bites after a recent hike in Georgia. Molecular methods later identified Ehrlichia chaffeensis as the etiologic agent. The cytoplasmic inclusions seen were most likely in: A Meningeal epithelial cells B Schwan cells C Polymorphonuclear leukocytes D Granulocytes E Mononuclear leukocytes
ANSWER: E - The disease is called human monocytic ehrlichiosis. Answer analysis: A. Not involved B. Not involved C. May be present by not with inclusions D. The target for a similar disease, human granulocytic anaplasmosis E. CORRECT - Transmitted by ticks from a deer reservoir
Over half the persons attending a banquet developed a vague febrile illness 2 to 5 days later. Most of those who sought medical aid had the same organism isolated from their bloodstream. Food histories incriminate dairy products but it appears certain they were kept refrigerated right up to the serving time. Which of the following is most likely to be the cause? A Escherichia coli O:157:H7 B Salmonella enterica C Salmonella serotype Typhi D Clostridium perfringens E Listeria monocytogenes
ANSWER: E - The key is only Listeria grows at refrigerator temperatures Answer analysis: A. Causes bloody diarrhea B. Usually at least some diarrhea C. Typhoid fever does not begin until weeks after ingestion D. Diarrhea primary manifestation (only some strains) E. CORRECT - Listeria diagnosis will be missed without a blood culture because there are no findings other than fever.
A research microbiologist is said to have caused a fatal pneumonia by sending bacterial spores through the mail. These spores are: A Structures for sexual reproduction B Packets of toxin C Concentrated peptidoglycan D Concentrated endotoxin E Inert survival forms
ANSWER: E - The spores can germinate to produce metabolically active vegetative cells. Answer analysis: A. Bacteria do not reproduce sexually B. The spores themselves are non-toxic C. Not present D. Not present E. CORRECT - These Bacillus anthracis spores were inhaled and germinated in-vivo producing pulmonary anthrax.
A human cell has been brought in contact with a bacterial A/B toxin. The cell type is known to be susceptible to the toxin. The most important determinant of the type of physiologic effect of the toxin on the cell is: A Surface binding receptors B Ribosomal receptor sites C Endocytotic vacuole D Type of enzymatic reaction E Function of target protein
ANSWER: E - This determines what physiologic function of the bacterial will be altered. Answer analysis: A. Determines whether the toxin can act, not its action. B. Same as above. C. This is a mechanism for entry into the cell D. The same enzymatic reaction can produce multiple effects. E. CORRECT - Can stimulate or inhibit.
An emergency call to a neighbor leads to an entire family with apparent paralysis of ocular and respiratory muscles. They had just embarked on a project of home canning and had consumed one of their own products (green beans) the evening before. It is most likely they consumed a toxin which: A Stimulates neuromuscular synapses B Stimulates neurotransmission in the spinal cord C Blocks postsynaptic inhibition in the spinal cord D Blocks epinephrine release in the spinal cord E Blocks acetylcholine release at neuromuscular junctions
ANSWER: E - This is a case of botulism due to the ingestion of toxin preformed in the beans Answer analysis: A. This might cause contraction but not paralysis B. Nonspecific C. This causes the hyper-contractions of tetanus D. Epinephrine not involved in contractions E. CORRECT - The ingested toxin reached the neuromuscular junctions via the bloodstream.
An immunocompromised patient reported chest pains and weight loss. Sputum showed branching, filamentous gram-positive rods which were weakly acid-fast. This organism was most probably acquired from: A Oropharyngeal microbiota B Family member C Domestic pet D Insect vector E Soil
ANSWER: E - This is a case of pneumonia caused by Nocardia which is widely found in the environment particularly soil. Answer analysis: A. The only other branching gram-positive rod pathogen is Actinomyces which is not acid-fast B. Actinomyces are found in the intestinal microbiota. C. Nocardia not associated with insects or animals. D. Same as above E. CORRECT - Inhaled in dust aerosols.
Twelve hours after birth a newborn is lethargic and feeding poorly. A blood culture reveals gram-positive cocci in short chains which are catalase negative. The cell wall of this organism most certainly contains: A Pili B Flagella C Lipopolysaccharide D Outer membrane E Peptidoglycan
ANSWER: E - This is neonatal group B streptococcal sepsis. Answer analysis: A. Not a virulence factor for GBS. B. Same as above C. Only in gram negatives D. Same as above. E. Peptidoglycan is prominent is all gram-positive bacteria.
As part of an annual evaluation, a 30-year-old medical resident's tuberculin skin test shows 16 mm induration. She has always had negative tests in the past including one a year earlier. She is afebrile, feels well, and her chest X-ray is clear. The best course of action at this point is: A Sputum acid-fast stain B Sputum TB culture C Bronchoalveolar lavage (BAL) with culture D 3 drug TB therapy E Isoniazid chemoprophylaxis
ANSWER: E - This will reduce the chances of evolution to active tuberculosis. Answer analysis: A. Useful only if clinical or radiologic signs of disease are present. B. Same as above C. Same as above D. Too aggressive with no signs of disease E. CORRECT - Prevention is most successful with a recent skin test converter.
A few days after returning from vacation in a developing country, multiple members of a family developed diarrhea. Routine stool cultures were negative, but a molecular panel detected an E. coli strain associated with intestinal disease. If the mucosa shows pedestal-like extensions and the stool shows blood this strain most likely produces: A Shiga toxin (Stx) B Labile toxin (LT) C Attaching and effacing (A/E) lesions D Invasion, ulcers E A and B F A and C
ANSWER: F - This is enterohemorrhagic E. coli which carries a significant mortality rate. Answer analysis: A. Combined with A/E lesions in enterohemorrhagic E. coli. B. Watery diarrhea, no blood. C. Enteropathogenic E. coli D. Stool would show blood and WBCs E. Not seen together F. CORRECT - The cause of many restaurant-associated outbreaks.
A man presents to urgent care with a history of fever, a shaking chill, and the production of reddish-colored sputum. The X-ray shows consolidation of the right middle lobe of the lung, and a Gram stain of the sputum shows numerous neutrophils and lancet-shaped gram-positive diplococci. External to the cell wall of this organism is typically found: A Flagella B Pili C Lipopolysaccharide D Polysaccharide E Exotoxin
Explanation: ANSWER: D - This is pneumococcal pneumonia involving a polysaccharide capsule. Answer analysis: A. Not a virulence factor for S. pneumoniae. B. Same as above. C. Only present in gram-negatives. D. CORRECT - The pneumococcus is avirulant without its capsule. E. Pneumolysin may contribute to lung symptoms.
A middle-aged man has developed a cough and rapidly rising fever. A chest radiograph shows pulmonary interstitial infiltrates and some consolidation. A Gram stain of a lung aspirate is unrevealing, but culture on a special medium buffered to an acidic pH yields gram-negative rods. The original source of this infection is most likely: A Human carrier B Insect C Animal D Water E Dust
This is Legionnaires' disease with the L. pneumophila originally derived from a water associated habitat. Answer analysis: A. No human to human transmission in Legionnaires' B. No insects involved. C. No animals but amoebas aid environmental survival D. CORRECT - Need more information to determine the water source. E. Not involved.
