USMLE Microbiology Prt 9
Questions and Answers
- 1.
A 1-year-old child develops voluminous watery diarrhea and vomiting. She is brought to the pediatrician by her parents and evaluated, then sent home with instructions for the parents to give the child an electrolyte replacement solution. Which of the following viruses is the most likely cause of the child's diarrhea?
- A.
Coronavirus
- B.
Lymphocytic choriomeningitis virus
- C.
Norwalk agent
- D.
Orbivirus
- E.
Rotavirus
Correct Answer
E. RotavirusExplanation
The correct answer is E. Rotavirus is the major cause of diarrhea in infants and children under
the age of 2. The replicates in the intestinal mucosa, producing a profuse, watery, non-bloody
diarrhea, often coupled with nausea and vomiting. Transmission is by the fecal-oral route.
Coronaviruses (choice A) usually cause cold-like illnesses.
Lymphocytic choriomeningitis virus (choice B) can cause headache, malaise, myalgia,
conjunctivitis, and, occasionally, meningitis.
Norwalk agent (choice C) can also cause diarrhea, but usually affects patients older than 2
years.
Orbivirus (choice D) is the cause of Colorado tick fever, which is the only tick-borne viral
disease in the United States.Rate this question:
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- 2.
A 3-year-old child with cystic fibrosis presents with weight loss, irritability, and a chronic productive cough. On physical exam, he is febrile and lung exam reveals intercostal retractions, wheezing, rhonchi, and rales. Chest x-ray demonstrates patchy infiltrates and atelectasis and Gram's stain of the sputum reveals slightly curved, motile gram-negative rods that grow aerobically. The microorganism responsible for this child's pneumonia is also the most common cause of which of the following diseases?
- A.
Croup
- B.
Epiglottitis
- C.
Meningitis
- D.
Otitis externa
- E.
Otitis media
Correct Answer
D. Otitis externaExplanation
The correct answer is D. Anytime you see pneumonia in a cystic fibrosis patient you should
suspect Pseudomonas aeruginosa. The Gram's stain revealing aerobic, gram-negative rods confirms
your suspicion in this case. Now the question is: which of the diseases listed is also caused
by Pseudomonas? The answer is otitis externa. P. aeruginosa is often found in the external ear,
especially if the conditions are moist ("swimmer's ear") and there is any sort of inflammation.
External otitis is usually a benign process with the only symptoms being an itchy, painful ear.
If, however, the organism penetrates the epithelium and invades the soft tissue, cartilage, and
cortical bone, the process becomes malignant otitis externa, which can progress to
osteomyelitis leading to cranial nerve palsies. This condition is most common in diabetics.
Croup (choice A), also called laryngotracheobronchitis, is a respiratory disease of children
that presents with a characteristic "barking" cough. Croup is caused by parainfluenza virus.
Epiglottitis (choice B) is a potentially fatal infection in children, caused by H. influenzae,
which presents with drooling, difficulty breathing, and stridor. The incidence of this disease
has dropped dramatically with the introduction of the H. influenzae type b (Hib) vaccine.
Meningitis (choice C) is caused by numerous different bacteria, depending on the age of the
patient. The most common causes include S. pneumoniae (elderly), H. influenzae (unvaccinated
children), Group B Strep and E. Coli (neonates) and N. meningitidis (1 month - adult). While P.
aeruginosa can cause meningitis, it is not a common cause.
The most common causes of otitis media (choice E) include S. pneumoniae and H. influenzae. Even
in cases of external ear infections with P. aeruginosa, the middle ear is typically spared.Rate this question:
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- 3.
A pastry chef cut his finger while slicing a cake. After a week, the site of the injury is warm, red, and swollen, and begins draining pus. While preparing some cream pies, he contaminates the custard with drainage from the lesion. The pies were eaten several days later by patrons of the restaurant. Within 4 hours they developed diarrhea and vomiting with no fever. Which of the following organisms would be most likely to cause these symptoms?
- A.
Bacillus cereus
- B.
Clostridium perfringens
- C.
Escherichia coli
- D.
Shigella sonnei
- E.
Staphylococcus aureus
Correct Answer
E. Staphylococcus aureusExplanation
The correct answer is E. The chef had a staphylococcal abscess on his finger. S. aureus,
produces enterotoxin A, which was likely present in the cream pies. When ingested, the toxin
causes severe nausea and vomiting within a few hours (the average incubation time is 3-6
hours). There is little diarrhea associated with this type of food poisoning outbreak.
Bacillus cereus(choice A) is a gram-positive spore-forming rod that is associated with food
poisoning outbreaks following the ingestion of fried rice. The time of onset and symptoms would
mimic staphylococcal disease; the major differentiating feature is the food involved. The
organism survives the boiling of the rice because it is a spore-former. It germinates as the
rice cools, grows, and elaborates an enterotoxin that is responsible for the nausea and
vomiting characteristic of the disease.
Clostridium perfringens(choice B) is a gram-positive spore-forming anaerobe that can cause a
longer incubation (18-24 hour) food poisoning, typically with marked diarrhea. Once again, the
spores allow the organism to survive the heating process used in the preparation of the food.
Both Clostridium perfringens and Clostridium botulinum are associated with home-canned
vegetable and sausages.
The symptoms of Escherichia coli(choice C) food poisoning are usually watery diarrhea
(traveler's diarrhea) with minimal nausea and vomiting, or a bloody diarrhea caused by
enteroinvasive strains of the agent. Also, E. coli would be an unlikely cause of the primary
infection in the chef.
Shigella sonnei (choice D) causes enterocolitis characterized by fever, cramps, and diarrhea
after an incubation period of one to four days. Transmission is fecal-oral, associated with
poor hygiene. A wide range of foods has been implicated.Rate this question:
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- 4.
After passing his physical exam, a 19-year-old army recruit gives urine and blood samples for further testing. Serum analysis yields elevated ALT, HBsAg, Anti-HBc, HBeAg, and bilirubin. All other values are normal. Which of the following is the hepatitis B status of this recruit?
- A.
Asymptomatic carrier
- B.
Chronic active carrier
- C.
Fulminant hepatitis B
- D.
Recovered from acute self-limited HBV
- E.
Vaccinated against HBV
Correct Answer
B. Chronic active carrierExplanation
The correct answer is B. The presence of elevated ALT, HBsAg, anti-HBc, HBeAg, and bilirubin
all point to active hepatitis B.
An asymptomatic carrier (choice A) does not have elevated ALT and bilirubin.
The absence of findings on physical examination rules out fulminant hepatitis B (choice C).
Recovery from acute self-limited HBV (choice D) is associated with the presence of anti-HBs and
the decrease in HBsAg and HBeAg. Someone who is vaccinated with HBV (choice E) has anti- HBs only in their serumRate this question:
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- 5.
A 4-year-old child living in a slum is bitten by a rat while sleeping. Two days later, the child develops a rash characterized by discrete erythematous 1-4 mm macules on the extremities and face, most obvious on the palms and soles. Which of the following organisms is the most likely cause of this child's disease?
- A.
Borrelia burgdorferi
- B.
Pseudomonas mallei
- C.
Pseudomonas pseudomallei
- D.
Spirillium minus
- E.
Streptobacillus moniliformis
Correct Answer
E. Streptobacillus moniliformisExplanation
The correct answer is E. The child has "rat-bite fever." This occurs in two forms with somewhat
similar clinical manifestations. The form this child has is the Haverhill fever form, caused by
Streptobacillus moniliformis and characterized by a short (often 1-3 days) incubation period.
The Haverhill form is more common in the United States than the Sodoku form, which is caused by
Spirillium minus, has a 1-4 week incubation period, and is most prevalent in Japan. One of the
problems with diagnosing these diseases is that the victims are usually young children, and the
bite site may be inapparent by the time the disease becomes severe enough for the child to be
taken to a doctor.
Borrelia burgdorferi(choice A) causes Lyme disease, which is characterized by an expanding
erythematous rash, arthralgias, and eventual nervous system involvement.
Pseudomonas mallei(choice B) causes glanders, which generally affects horses or humans in close
contact with equines.
Pseudomonas pseudomallei(choice C) causes melioidosis, a rare pulmonary disease found mostly in
Southeast Asia.
Spirillium minus(choice D) causes the Sodoku form of rat-bite fever.Rate this question:
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- 6.
A 35-y/o man develops hemiparesis, ataxia, homonymous hemianopia, and cognitive deterioration. An MRI of the brain demonstrates widespread areas of abnormal T2 signal in the white matter. An electroencephalogram is remarkable for diffuse slowing over both cerebral hemispheres. Brain biopsy reveals demyelination with abnormal giant oligodendrocytes, some of which contain eosinophilic inclusions. This patient's condition is most closely related to which of the following diseases?
- A.
AIDS
- B.
Chickenpox
- C.
Measles
- D.
Syphilis
- E.
Tuberculosis
Correct Answer
A. AIDSExplanation
The correct answer is A. The condition is progressive multifocal leukoencephalopathy, which is
a rapidly progressive demyelinating disorder in which the JC virus (a papovavirus) infects
oligodendroglial cells in the brain. The eosinophilic inclusions represent accumulations of JC
virus. PML occurs in about 1% of AIDS patients, and is the AIDS-defining illness in half of the
patients who develop the condition. There is no effective treatment for this disorder.
Shingles and post-infectious encephalitis can follow chickenpox (choice B), but varicella is
not associated specifically with demyelination.
Measles (choice C) can cause an encephalitis, and in some cases, subacute sclerosing
panencephalitis (SSPE) may follow previous measles infection. These disorders would affect not
only white matter, but gray matter as well. Also, SSPE usually occurs before the age of 18.
Syphilis (choice D) can cause meningitis, encephalitis, and spinal cord damage, but the disease
process would not be limited to the white matter.
Tuberculosis (choice E) can cause a granulomatous meningitis, typically affecting the base of
the brain.Rate this question:
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- 7.
An otherwise healthy patient who has just received a prosthetic aortic valve develops postoperative fever. Blood cultures are done and she is placed on broad-spectrum antibiotics. Two days later she is still febrile and clinically deteriorating. Which of the following organisms is the most probable etiologic agent?
- A.
Actinomyces israelii
- B.
Candida albicans
- C.
Histoplasma capsulatum
- D.
Nocardia asteroides
- E.
Trichophyton rubrum
Correct Answer
B. Candida albicansExplanation
The correct answer is B. The patient likely has a candidal infection of the prosthetic aortic
valve. That is why she did not respond favorably to antibacterial therapy, which is known to
promote fungal infection. Note that Candida tends to colonize foreign bodies such as IV and
Foley catheters, prosthetic valves, and ventricular shunts.
Actinomyces israelii(choice A) is known to cause cervicofacial infections in patients having
undergone dental work.
Histoplasma capsulatum(choice C) causes histoplasmosis, a pulmonary infection common in the
midwestern river valleys. Multi-organ involvement is usually seen only in the
immunocompromised. Transmission of the organism occurs through the inhalation of airborne
microconidia (infectious) spores. The organism is found in bird and bat droppings and in the
soil.
Nocardia asteroides(choice D) is an actinomycete that causes a chronic lobar pneumonia that may
metastasize to the brain. It is more common in the immunocompromised. It is found in soil and
aquatic environments.
Trichophyton rubrum(choice E) is one of the organisms that commonly produces a variety of
cutaneous mycoses, including tinea corporis (ringworm), tinea cruris (jock itch), and tinea
pedis (athlete's foot).Rate this question:
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- 8.
An 18-year-old college student presents to the student health center complaining of a sore throat and fever. He describes feeling tired for the past few days and reports a loss of appetite. On examination, he has pharyngitis with cervical lymphadenopathy. Blood tests reveal lymphocytosis and the presence of heterophil antibodies. Which of the following best describes the virus responsible for his illness?
- A.
Double-stranded, enveloped DNA virus
- B.
Double-stranded, nonenveloped DNA virus
- C.
Single-stranded, enveloped RNA virus
- D.
Single-stranded, nonenveloped DNA virus
- E.
Single-stranded, nonenveloped RNA virus
Correct Answer
A. Double-stranded, enveloped DNA virusExplanation
The correct answer is A. This is one of those USMLE-style items where figuring out the
diagnosis is the easy part and remembering the basic science details is much tougher. In this
case, the patient has all the hallmarks of mononucleosis (the heterophil antibodies should have
confirmed your suspicion from the history and physical). Mononucleosis is caused by the
Epstein-Barr virus, which, in turn, belongs to the herpesvirus family. The herpesviruses are
enveloped viruses with double-stranded DNA. Remember that in addition to Epstein-Barr virus,
the herpesvirus family also includes herpes simplex (1 and 2), varicella-zoster (chickenpox,
shingles), and cytomegalovirus (infection in immunocompromised). Cytomegalovirus also causes
infectious mononucleosis, but in these patients the heterophil test is negative.
There are two families of viruses that are nonenveloped with double-stranded DNA (choice B):
papovaviruses and adenoviruses.
There are many families of viruses that are enveloped with single-stranded RNA (choice C):
arenaviruses, bunyaviruses, coronaviruses, filoviruses, flaviviruses, paramyxoviruses,
orthomyxoviruses, retroviruses, rhabdoviruses, and togaviruses.
Parvoviruses are the only family of DNA virus with single-stranded DNA. They do not have an
envelope (choice D).
There are two families of RNA virus that are single-stranded without an envelope (choice E):
caliciviruses and picornaviruses.Rate this question:
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- 9.
A 4-year-old African child develops an infection with Chlamydia trachomatis. How does infection with this organism cause blindness?
- A.
Cataract formation
- B.
Hemorrhage into the anterior chamber
- C.
Hemorrhage into the posterior chamber
- D.
Retinal detachment
- E.
Scarring of the cornea
Correct Answer
E. Scarring of the corneaExplanation
The correct answer is E.Chlamydia trachomatis (serotypes A, B, and C) causes a conjunctival and
corneal infection that is spread in developing countries by eye-seeking flies. The lesions
begin with formation of lymphoid follicles in the conjunctiva. With disease progression, there
is tissue necrosis, granulation tissue deposition, and scar formation, leading to lacrimal duct
obstruction and distortion of the eyelids. With the loss of an adequate tear system, the cornea
becomes vulnerable to dehydration and opacification. Also, the vigorous inflammatory response
can directly involve the cornea, with resulting opacity. In developed countries, chlamydial eye
infections are often transmitted venereally rather than by flies, and may cause conjunctivitis
in the newborn and in sexually active young adults.Rate this question:
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- 10.
A mother brings her 3-year-old boy into the emergency room because he has developed a harsh, "barking" cough with hoarseness. The virus responsible for this child's illness belongs to which of the following families?
- A.
Papovavirus
- B.
Paramyxovirus
- C.
Parvovirus
- D.
Picornavirus
- E.
Poxvirus
Correct Answer
B. ParamyxovirusExplanation
The correct answer is B. This question is difficult for two reasons—it asks for the viral
family instead of the virus itself, and it includes distracters that all look alike. You
probably realized that the child in question suffers from croup (laryngotracheobronchitis); the
classic clue here is the "barking cough." You might have remembered that the virus responsible
for croup is the parainfluenza virus. The toughest part was remembering that parainfluenza
virus (along with measles virus, mumps virus, and respiratory syncytial virus) belongs to the
paramyxovirus family. These viruses all have negative-strand RNA and an enveloped helical
nucleocapsid.
The papovaviruses (choice A) are DNA viruses with a naked icosahedral nucleocapsid. This family
includes the human papilloma viruses (which cause warts and are associated with penile,
laryngeal, and especially, cervical cancer), the BK virus that can affect immunosuppressed
patients, the JC virus associated with progressive multifocal leukoencephalopathy, and the
simian SV40 virus.
The parvoviruses (choice C) are small, single-stranded DNA viruses. Only one serotype (B19)
causes diseases in humans. It causes erythema infectiosum in children (characteristic "slapped
cheek" rash), aplastic crises in patients with hemolytic diseases, and hydrops fetalis or
stillbirth in anemic fetuses.
The picornaviruses (choice D) are positive single-stranded RNA viruses with a naked icosahedral
nucleocapsid. This family includes the polioviruses, echoviruses, coxsackieviruses, enterovirus
72 (HepA virus), and the rhinoviruses (common cold).
The poxviruses (choice E) are double-stranded DNA viruses. This family includes the viruses
responsible for smallpox and molluscum contagiosum.Rate this question:
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- 11.
A 24-year-old man presents with fever, rash, a mild headache, and a sore throat. He denies HIV risk factors, although he is sexually active. On examination, his temperature is 100.8°F and his pulse is 90/min. There is a diffuse, erythematous, maculopapular rash over most of his body. Generalized adenopathy is appreciated, and photophobia is noted when funduscopic examination is attempted. If this man is not treated, which of the following changes in his serologic status will most likely occur?
- A.
The FTA-ABS titer would fall
- B.
The FTA-ABS titer would rise
- C.
The VDRL titer would fall
- D.
The VDRL titer would rise
- E.
Both FTA-ABS and VDRL would fall
- F.
Both FTA-ABS and VDRL would rise
Correct Answer
C. The VDRL titer would fallExplanation
The correct answer is C. This is a case of syphilis, which is diagnosed serologically, using
either treponemal or nontreponemal tests. The FTA-ABS (fluorescent treponemal antibody-absorbed
test) is the most widely used of the specific tests and depends on fluorescent labeling of the
organisms with anti-treponemal antibody. The specific tests tend to rise early in titer and
stay elevated throughout the lifetime of the host, or at least until well after drug therapy
has been completed. The VDRL (Venereal Disease Research Laboratory) is a nontreponemal test
that detects antibodies that cross-react with mammalian cardiolipid called reaginic antibodies.
This test will become positive after the specific test, and its titer will fall late in
infection, with or without drug therapy.
The FTA-ABS titer would fall (choice A) is incorrect because this titer will remain high
throughout the life of the host, falling off only very slowly after drug cure.
The FTA-ABS titer would rise (choice B) is incorrect because the specific antibody test will
reach a high positive titer and remain at that level in untreated late stage syphilis.
The VDRL titer would rise (choice D) is incorrect because reaginic antibody levels will fall in
late stage syphilis, with or without drug treatment.
Both FTA-ABS and VDRL would fall (choice E) is incorrect because specific antibody levels would
remain high, while reaginic antibodies would fall.
Both FTA-ABS and VDRL would rise (choice F) is incorrect because specific antibody levels would
remain high, while reaginic antibodies would fall.Rate this question:
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- 12.
A 22-year-old male military recruit complains of a headache and stiff neck. He is examined, blood is drawn, and a lumbar puncture performed. The glucose in the CSF is 100 mg/dL and the serum glucose is 120 mg/dL. The CSF shows 3 lymphocytes and 0 neutrophils/microliter. Which of the following conclusions concerning the interpretation of these findings is most accurate?
- A.
The CSF glucose level suggests bacterial meningitis
- B.
The CSF glucose level suggests viral meningitis
- C.
The lymphocytes suggest bacterial meningitis
- D.
The lymphocytes suggest viral meningitis
- E.
There is no evidence for meningitis
Correct Answer
E. There is no evidence for meningitisExplanation
The correct answer is E. Both the CSF glucose level and the small number of lymphocytes present
are within normal limits. It is normal for the CSF glucose to be less than serum glucose, often
about 2/3 of the serum value.
Bacterial meningitis (choices A and C) can profoundly lower CSF glucose levels. CSF from
bacterial meningitis cases also usually shows large numbers of neutrophils.
Viral meningitis (choices B and D) will not alter the CSF glucose level, but the CSF often
contains many lymphocytes.Rate this question:
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- 13.
Autopsy of a 23-year-old male victim of a motor vehicle accident reveals a small cluster of caseating granulomas in the right lung just above the interlobar fissure and similar granulomas in the hilar lymph nodes. Acid-fast staining demonstrates acid-fast bacilli within these lesions. No other lesions were found in the remaining organs and systems. Which of the following is the most accurate interpretation of these findings?
- A.
Cavitary tuberculosis
- B.
Ghon complex
- C.
Histoplasma infection
- D.
Miliary tuberculosis
- E.
Remote healed tuberculosis
Correct Answer
B. Ghon complexExplanation
The correct answer is B. The Ghon complex is the most frequent pathologic form of primary
pulmonary tuberculosis. Mycobacterium tuberculosis first localizes in the lung parenchyma, then
in the hilar lymph nodes. In both these locations, a granulomatous reaction takes place. These
lesions usually heal by fibrosis, leaving only small scars at the sites of remote tuberculous
infection. In some cases, owing to immunosuppression (e.g., AIDS, immunosuppressant treatment,
and lymphomas), reactivation of dormant bacilli in old lesions or additional re-exposure leads
to secondary tuberculosis, with progression of lesions. Sometimes, active lesions of the Ghon
complex are discovered by chance at autopsy. More frequently, scars due to remote healed
tuberculosis (choice E) are found postmortem and listed in autopsy reports as incidental
findings
Cavitary tuberculosis (choice A) and miliary tuberculosis (choice D) are expressions of
secondary infection, following reactivation of old, usually clinically silent, lesions. The
cavitary form is characterized by development of large areas of liquefactive necrosis that
empty into the airspaces, leading to cavities within the lung parenchyma. The miliary form is
due to lymphohematogenous dissemination and subsequent seeding of tubercle bacilli throughout
the body, with myriad small granulomas forming in the lungs, spleen, liver, bone marrow,
retina, and adrenals, for example.
Acid-fast bacilli suggest tuberculosis, rather than an infection with a fungus such as
Histoplasma(choice C).Rate this question:
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- 14.
A 53-year-old woman with diarrhea and lower abdominal pain of 3 days duration comes to her physician after failing to relieve her symptoms with various home remedies. She denies any recent travel and states that there is blood and pus in her stool. Fecal cultures yield several flagellated, curved, oxidase-positive, gram-negative rods. The organism isolated is most likely
- A.
Campylobacter jejuni
- B.
Escherichia coli
- C.
Salmonella
- D.
Shigella
- E.
Vibrio cholera
Correct Answer
A. Campylobacter jejuniExplanation
The correct choice is A. Campylobacter is a motile, curved, oxidase-positive, gram-negative rod
with a polar flagella. The illness typically begins 1 to 7 days following ingestion of the
organism. The presentation is usually lower abdominal pain and diarrhea with blood and pus. The
illness is self-limited after 3 to 5 days and can last up to 2 weeks. The organisms grow
optimally at 42° C under microaerophilic conditions.
Escherichia coli(choice B), although a flagellated gram-negative organism, is not the correct
choice since it is not as common a cause of bloody diarrhea in this age group and is not
oxidase positive.
Salmonella (choice C) is incorrect because it is oxidase negative.
Shigella (choice D) is incorrect because it is oxidase negative.
Vibrio cholera(choice E) has many physical features in common with C. jejuni. However, V.
cholera is not enteroinvasive and does not produce bloody diarrhea.Rate this question:
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- 15.
A hospitalized patient develops dysuria and suprapubic pain and is treated with ciprofloxacin. What is the mechanism of action of this antibiotic?
- A.
It inhibits dihydrofolate reductase
- B.
It inhibits DNA-dependent RNA polymerase
- C.
It inhibits protein synthesis by binding to the 30s ribosomal subunit
- D.
It inhibits protein synthesis by binding to the 50s ribosomal subunit
- E.
It inhibits topoisomerase II (DNA gyrase)
Correct Answer
E. It inhibits topoisomerase II (DNA gyrase)Explanation
The correct answer is E. This is a straightforward question where the introductory clinical
details are really irrelevant. Ciprofloxacin and norfloxacin belong to a category of antibiotics
called the fluoroquinolones. They are bactericidal and work by inhibiting topoisomerase II (DNA
gyrase). They are effective against gram-negative rods and are the only oral agents effective
against Pseudomonas. Ciprofloxacin is effective for treating UTIs, gonorrhea, diarrheal
diseases, and soft tissue infections. It is also used to treat Pseudomonas infections in cystic
fibrosis.
The wrong answer choices provide us with the opportunity to discuss important mechanisms of
action for other antimicrobials:
Inhibition of dihydrofolate reductase (choice A) is the mechanism of action of trimethoprim.
Trimethoprim is typically used in combination with sulfonamides (trimethoprim-sulfamethoxazole).
Sulfonamides inhibit an earlier step in folate synthesis (dihydropteroate synthase), so the
combination with trimethoprim is an effective "one-two" punch. Trimethoprim-sulfa is used in the
treatment of Shigella, Salmonella, recurrent UTIs, and in Pneumocystis carinii pneumonia.
Inhibition of DNA-dependent RNA polymerase (choice B) is the mechanism of action of rifampin.
Rifampin is used (along with other drugs) in the treatment of tuberculosis. You should also
remember that rifampin can be used to treat individuals exposed to Meningococcus or H.
influenzae type B.
Inhibition of the 30s ribosomal subunit (choice C) is the mechanism of action of two important
classes of antibiotic—the tetracyclines (tetracycline, doxycycline, demeclocycline) and
the aminoglycosides (gentamicin, tobramycin, streptomycin, etc.). The tetracyclines inhibit the
attachment of the aminoacyl-tRNA to the ribosome while the aminoglycosides inhibit the formation
of the initiation complex.
Inhibition of the 50s ribosomal subunit (choice D) is the mechanism of action of the macrolides
(e.g., erythromycin), the lincosamides (e.g., lincomycin, clindamycin), and chloramphenicol.
Chloramphenicol inhibits the 50s peptidyl transferase, while erythromycin blocks translocation.Rate this question:
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- 16.
Cefuroxime is believed to exert its antibacterial effect by which of the following mechanisms?
- A.
Competitive inhibition of para-aminobenzoic acid (PABA)
- B.
Inhibition of bacterial cell wall synthesis
- C.
Inhibition of DNA-gyrase
- D.
Irreversible binding to the 30S subunit of bacterial ribosomes
- E.
Irreversible binding to the 50S subunit of bacterial ribosomes
Correct Answer
B. Inhibition of bacterial cell wall synthesisExplanation
The correct answer is B. Cephalosporins, such as cefuroxime, are believed to exert their
antibacterial effect by binding to one or more of the penicillin-binding proteins located on the
cell walls of susceptible organisms. This action results in the inhibition of the third, and
final, stage of bacterial cell wall synthesis. This is also the mechanism of action of the
penicillins. Cefuroxime is a second generation cephalosporin used to treat infections in the
lower respiratory and urinary tracts, as well as otitis media. It is also efficacious in the
treatment of gonorrhea and is used for perioperative prophylaxis in various surgical procedures,
such as coronary artery bypass grafting.
Sulfonamide antibiotics, such as sulfamethoxazole-trimethoprim, exert their antibacterial effect
through the competitive inhibition of para-aminobenzoic acid (PABA) (choice A), thereby
inhibiting folic acid biosynthesis required for bacterial growth.
Quinolone antibiotics, such as ciprofloxacin, inhibit DNA-gyrase (choice C), which is an enzyme
necessary for bacterial DNA replication and repair.
Aminoglycosides, such as gentamicin, irreversibly bind to the 30S subunit of bacterial ribosomes
(choice D), inhibiting bacterial protein synthesis.
Lincosamides, such as clindamycin, irreversibly bind to the 50S subunit of bacterial ribosomes
(choice E), suppressing bacterial protein synthesis. Note that macrolides, such as erythromycin,
reversibly bind to the 50S subunit of bacterial ribosome.Rate this question:
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- 17.
A 19-year-old college student presents to the student health clinic complaining of weakness, malaise, and a chronic cough. He has a fever of 100 degrees F and a dry cough; no sputum can be obtained for laboratory analysis, so a bronchial lavage is performed and the washings are submitted to the laboratory. The laboratory reports that the organism is "slow-growing." Serodiagnosis reveals Strep MG agglutinins in the patient's serum. Which of the following organisms is the most likely cause of this student's illness?
- A.
Klebsiella pneumoniae
- B.
Mycoplasma pneumoniae
- C.
Parainfluenza virus
- D.
Respiratory syncytial virus
- E.
Streptococcus pneumoniae
Correct Answer
B. Mycoplasma pneumoniaeExplanation
The correct answer is B. The patient has primary atypical pneumonia caused by Mycoplasma
pneumoniae. This organism is fastidious and difficult to culture in the laboratory, however
serodiagnosis can be most helpful. Patients typically produce one or two heterophile antibodies
during the course of the infection; one agglutinates human O+ RBCs in the cold (the cold
hemagglutinin) while the other causes the agglutination of a strain of Streptococcus salivarius
termed strain MG (the Strep MG agglutinins).
Klebsiella pneumoniae(choice A) is readily cultured on routine laboratory media and
characteristically produces pneumonia with blood clots in the sputum (red currant jelly
sputum), which may be indicative of pulmonary abscess development.
Parainfluenza viruses (choice C) cause croup, which is characterized by a dry, "barking" cough.
It is more of a tracheitis, bronchitis, and bronchiolitis than a pneumonitis. No heterophile
antibodies are produced in these patients.
Respiratory syncytial virus (choice D) causes an atypical pneumonitis in infants. It is usually
diagnosed by the observation of syncytial masses in respiratory secretions. Cold hemagglutinins
and Strep MG agglutinins are absent.
Streptococcus pneumoniae (choice E) is the number one cause of pneumonia in adults. It also
causes septicemia and meningitis in the elderly. The patient has a classical acute pneumonia
with a productive cough, high fever with chills, leukocytosis, tachycardia, rapid respirations
and other signs of serious respiratory disease. A vaccine, composed of the capsular
carbohydrate of 23 serotypes of this organism, is routinely given to individuals over the age
of 60, as well as to individuals with splenic abnormalities (e.g., sickle cell disease) who are
at increased risk for pneumococcal sepsis.Rate this question:
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- 18.
An 8-month-old baby presents to the emergency room with a 1-day history of poor feeding and generalized weakness. The mother states that she often feeds the baby honey to pacify her. The toxin responsible for this presentation works by which of the following mechanisms?
- A.
It blocks the release of acetylcholine from the nerve terminal
- B.
It blocks the release of inhibitory neurotransmitters such as glycine and GABA
- C.
It has a subunit that inactivates an elongation factor by ADP-ribosylation
- D.
It is a lecithinase
- E.
It stimulates guanylate cyclase
Correct Answer
A. It blocks the release of acetylcholine from the nerve terminalExplanation
The correct answer is A. The clinical history suggests infant botulism. The clue here is that
the mother feeds the baby honey. Clostridium botulinum (a common honey contaminant) was
ingested and produced toxin in the infant. The toxin, which blocks the release of acetylcholine
from nerve terminals, is responsible for the floppiness. Acetylcholine is the neurotransmitter
at the neuromuscular junction and impairment of this can lead to muscle weakness, failure to
thrive, and in more serious cases, respiratory impairment.
Glycine and GABA (choice B) release is inhibited by C. tetani. This leads to the muscular
spasms of tetanus.
ADP-ribosylation of an elongation factor (choice C) is a mechanism of action of diphtheria and
Pseudomonas exotoxins.
Alpha toxin from C. perfringens is a lecithinase (choice D) responsible for the development of
gas gangrene.
Guanylate cyclase (choice E) is stimulated by a heat-stable toxin produced by E. coli.Rate this question:
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- 19.
A 7-year-old boy presents to the pediatrician because his mother noticed a "smoky" color to his urine. Upon questioning the mother, it is revealed that the child suffered a sore throat several weeks ago that was left untreated. Physical examination reveals hypertension and mild generalized edema. Urinalysis is significant for red blood cell casts. Which of the following accurately describes the microorganism responsible for this child's illness?
- A.
It causes alpha-hemolysis on blood agar
- B.
It is catalase positive
- C.
It is coagulase positive
- D.
It is sensitive to bacitracin
- E.
It is sensitive to optochin
Correct Answer
D. It is sensitive to bacitracinExplanation
The correct answer is D. This is one of the "higher order" questions the USMLE often favors. In
this case, you need to figure out what disease the child has, what organism causes the disease,
and which of the listed features is true of the microorganism. The disease in question is
poststreptococcal glomerulonephritis, as evidenced by the smoky urine, hypertension, edema, and
red blood cell casts in the urine sediment. The history of the prior sore throat is a tip-off
that this is a nonsuppurative sequela of an infection due to Streptococcus pyogenes (group A
β-hemolytic Streptococci). You must remember the features of S. pyogenes. S. pyogenes can
be differentiated from Streptococcus pneumoniae and viridans Streptococci by its hemolytic
pattern; it is beta-hemolytic, while the others are alpha-hemolytic (choice A). It can be
distinguished from the other beta-hemolytic Streptococci by its sensitivity to the antibiotic
bacitracin. Other important things to remember about S. pyogenes are its many virulence
factors, including M protein, antiphagocytic capsule, hyaluronidase, streptolysins O and S, and
erythrogenic toxins.
Choice B is incorrect because Streptococci are catalase negative, in contrast to Staphylococci,
which are catalase positive.
Choice C is incorrect because S. pyogenes is coagulase negative. (In fact, a coagulase test
should NOT be done on a catalase-negative organism.) The coagulase test is an important means
of differentiating Staphylococcus aureus, which are coagulase positive, from all other species
of Staphylococcus, which are coagulase negative.
Choice E is incorrect because optochin is used to differentiate the viridans Streptococci
(resistant) from S. pneumoniae (sensitive).Rate this question:
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- 20.
An AIDS patient with clinical pneumonia has a bronchoalveolar lavage that demonstrates small, "hatshaped" structures in alveoli that are about the size of an erythrocyte and stain with silver stains. The microorganism involved is most likely which of the following?
- A.
Aspergillus fumigatus
- B.
Blastomyces dermatitidis
- C.
Mycobacterium avium
- D.
Mycobacterium tuberculosis
- E.
Pneumocystis carinii
Correct Answer
E. Pneumocystis cariniiExplanation
The correct answer is E. This is the classic appearance of Pneumocystis cysts. Pneumocystis is
a common (and dangerous) cause of pneumonia in AIDS patients. The diagnosis is now frequently
made by bronchoalveolar lavage, which is much more effective at demonstrating the organism than
is either sputum or blood culture.
Aspergillus fumigatus(choice A) is a typical fungus, and hyphae would probably have been seen
in the lavage material.
Blastomycosis (choice B) is caused by a dimorphic fungus that grows in mammalian tissues as a
round, multinucleate, budding cell 8-15 micrometers in diameter.
Mycobacteria (choices C and D) are small, acid-fast rods.Rate this question:
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- 21.
Which of the following viruses is capable of replication in enucleated cells?
- A.
Adenovirus
- B.
Cytomegalovirus
- C.
Influenza virus
- D.
JC virus
- E.
Poliovirus
Correct Answer
E. PoliovirusExplanation
The correct answer is E. Most RNA viruses (eg, poliovirus) replicate in the cytoplasm and
therefore can replicate in enucleated cells. Poliovirus belongs to the family Picornaviridae.
These viruses are nonenveloped and have an icosahedral nucleocapsid that contains positivesense
RNA.
The exception to the rule regarding RNA viruses is the family Orthomyxoviridae, the influenza
viruses (choice C). Orthomyxoviruses undergo transcription and RNA replication in the nucleus
of the host cell because they need to cannibalize the capped 5' termini of cellular RNAs for
use as primers for viral mRNA transcription.
For most DNA viruses, transcription and DNA replication occur in the nucleus of the host cell.
The exception to this observation is the family Poxviridae, which carries out its replication
in the cytoplasm. Poxviridae includes variola virus, vaccinia virus, molluscum contagiosum, and
orf virus.
Adenoviruses (choice A) are nonenveloped and have an icosahedral nucleocapsid that contains a
double-stranded linear DNA genome.
Cytomegalovirus (choice B) is a member of family Herpesviridae. It is an enveloped virus with
an icosahedral nucleocapsid that contains a double-stranded linear DNA genome.
JC virus (choice D) belongs to family Papovaviridae. It is nonenveloped and has an icosahedral
nucleocapsid that contains a double-stranded circular DNA genome.Rate this question:
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- 22.
A 47-year-old man with a history of sickle cell disease has had numerous hospitalizations requiring the placement of intravenous lines. The patient has poor peripheral venous access, and a catheter is placed in the right subclavian vein. The patient subsequently develops right arm discomfort and swelling and a temperature of 40.1 degrees C with chills. Multiple blood cultures are taken, and gram-positive cocci are isolated. The organism is catalase positive and grows on mannitol salt agar, but does not turn the agar yellow; the colonies are gamma-hemolytic on a sheep blood agar plate. Which of the following organisms is the most likely cause of this patient's symptoms?
- A.
Enterococcus faecalis
- B.
Staphylococcus aureus
- C.
Staphylococcus epidermidis
- D.
Streptococcus agalactiae
- E.
Streptococcus pyogenes
Correct Answer
C. Staphylococcus epidermidisExplanation
The correct answer is C. The patient has developed bacteremia; the description of the causative
agent is consistent with a staphylococcal organism (catalase positive, gram-positive cocci that
grow on mannitol salt agar). The organism is most likely S. epidermidis as it was not able to
ferment mannitol, and was not hemolytic. Both of those characteristics tend to rule out S.
aureus(choice B). Two other tests that are commonly used are coagulase production and excretion
of DNAse from colonies. S. aureus is positive in both tests, S. epidermidis is negative.
Enterococcus faecalis(choice A) might grow on the mannitol salt agar as it is relatively
haloduric but these organisms are catalase negative. The enterococci are extremely variable in
hemolytic ability so this characteristic is not useful in species identification.
Both streptococcal organisms (choices D and E) are catalase negative and beta-hemolytic on
sheep blood agar plates. Also, neither would grow on the mannitol salt agar. S. pyogenes is
sensitive to growth inhibition by bacitracin while S. agalactiae (group B streptococci) is not.Rate this question:
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- 23.
Microbiologic studies on a gram-positive coccus isolated from a skin abscess demonstrate a positive catalase test and a positive coagulase test. This is most consistent with which of the following organisms?
- A.
S. aureus
- B.
S. epidermidis
- C.
S. pneumoniae
- D.
S. pyogenes
- E.
S. saprophyticus
Correct Answer
A. S. aureusExplanation
The correct answer is A. All of the organisms listed are gram-positive cocci. The positive
catalase test excludes the Streptococci, S. pyogenes(choice D), and S. pneumoniae(choice C). Of
the Staphylococci, only S. aureus has a positive coagulase test. S. epidermidis(choice B) and
S. saprophyticus(choice E) are coagulase-negative Staphylococci.Rate this question:
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Mar 20, 2023Quiz Edited by
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Nov 06, 2012Quiz Created by
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