USMLE Microbiology Prt 4

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USMLE Microbiology Prt 4 - Quiz

Questions and Answers
  • 1. 

    An Hispanic male is referred to the dermatology clinic of a major medical center. On physical examination, the man has several disfiguring lesions on his face and there is loss of cutaneous sensation to fine touch, pain, and temperature. An acid-fast organism is observed in scrapings from a skin lesion. Which of the following organisms is the most likely cause of this patient's disease?

    • A.

      Bartonella henselae

    • B.

      Listeria monocytogenes

    • C.

      Mycobacterium avium-intracellulare

    • D.

      Mycobacterium leprae

    • E.

      Nocardia asteroides

    Correct Answer
    D. Mycobacterium leprae
    Explanation
    The correct answer is D. The disease in question is leprosy, or Hansen's disease. A key feature in the
    description is the fact that the organism is acid-fast. Both of the mycobacteria, M. avium-intracellulare and M.
    leprae are strongly acid-fast, that is they retain the carbol fuchsin dye in the face of acid-alcohol decolorization.
    M. leprae has a predilection for the skin and cutaneous nerves, thereby producing the symptoms of
    depigmentation and anesthetic cutaneous lesions. This loss of peripheral nerve function leads to many of the
    disfiguring features of the disease; because the patients do not have normal pain sensation, they sustain
    repeated injuries. In addition, the organism attacks cartilage and causes granuloma formation in the skin,
    leading to some of the facial disfigurement.
    Bartonella henselae(choice A) is a very small, gram-negative bacterium that is closely related to the rickettsia,
    although it is able to be cultured on lifeless media. It is the cause of cat-scratch disease, a local, chronic
    lymphadenitis most commonly seen in children, and bacillary angiomatosis, a disease seen particularly in AIDS
    patients.
    Listeria monocytogenes(choice B) is a ubiquitous microbe that causes disease in over 100 animal species.
    Although it is best known as an agent of meningitis in the newborn, it is a cause of multiple other diseases. A
    characteristic feature of these infections is the development of granulomas at the site of the infection. The
    organism is not acid-fast and has no particular predilection for skin or nervous tissues.
    M. avium-intracellulare(choice C) causes tuberculosis-like pulmonary disease in the immunosuppressed.
    Nocardia asteroides(choice E) primarily produces pulmonary infections in humans. The organism is consider to
    be "weakly" acid-fast, meaning that if the amount of HCl used in the decolorization step is reduced, the
    organisms will retain the carbolfuchsin primary stain.

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  • 2. 

    A patient develops fever, shortness of breath, and appears to be quite ill. X-ray demonstrates bilateral interstitial lung infiltrates. Bronchial washings demonstrate small "hat-shape" organisms visible on silver stain within alveoli. Which predisposing condition is most likely to be present in this patient?

    • A.

      AIDS

    • B.

      Congestive heart failure

    • C.

      Pulmonary embolus

    • D.

      Rheumatoid arthritis

    • E.

      Systemic lupus erythematosus

    Correct Answer
    A. AIDS
    Explanation
    The correct answer is A. The disease is Pneumocystis pneumonia, which is caused by an agent now believed to
    be a fungus rather than a true bacteria. Pneumocystis carinii pneumonia is seen in immunocompromised
    patients, particularly in those with AIDS, cancer, and in malnourished children. It can be the AIDS-defining
    illness.
    Congestive heart failure (choice B) predisposes the patient to pulmonary edema.
    Pulmonary embolus (choice C) can cause pulmonary infarction or sudden death.
    Rheumatoid arthritis (choice D), particularly in miners, can cause formation of lung nodules similar to
    subcutaneous rheumatoid nodules.
    Systemic lupus erythematosus (choice E) can cause pleuritis, but is not associated with a significantly increased
    incidence of pneumonia.

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  • 3. 

    A 23-year-old man develops explosive watery diarrhea with blood, fecal leukocytes, and mucus approximately 3 days after eating chicken that was improperly cooked. Comma-shaped organisms were found in the fecal smear along with red blood cells and leukocytes. Which of the following pathogens is the most likely cause of these symptoms?

    • A.

      Campylobacter jejuni

    • B.

      Enterotoxigenic E. coli

    • C.

      Shigella sonnei

    • D.

      Staphylococcus aureus

    • E.

      Vibrio cholera

    Correct Answer
    A. Campylobacter jejuni
    Explanation
    The correct answer is A.Campylobacter jejuni is a pathogen causing an invasive enteric infection associated
    with ingestion of raw or undercooked food products, or through direct contact with infected animals. In the U.S.,
    ingestion of contaminated poultry that has not been sufficiently cooked is the most common means of acquiring
    the infection. The patients typically have bloody diarrhea, abdominal pain, and fever. The presence of fecal
    leukocytes indicates an invasive infection. The organism is a gram negative rod with a "comma-shape." Enterotoxigenic E. coli(choice B) causes the classic traveler's diarrhea. The infection is non-invasive and is
    acquired via the fecal-oral route through consumption of unbottled water or uncooked vegetables. The major
    manifestation is a copious outpouring of fluid from the GI tract presenting as explosive diarrhea. This is due to
    the action of one of two types of enterotoxins on the GI tract mucosa.
    Shigella sonnei(choice C) produces a syndrome very similar to C. jejuni. However, the organism appears as a
    gram-negative rod on Gram's stain. It does not have a comma shape. Transmission is from person to person
    via the fecal-oral route. Infection requires a low infective dose since the organism is fairly resistant to gastric
    acidity.
    Staphylococcus aureus(choice D) produces food poisoning due to the ingestion of a pre-formed enterotoxin.
    The organism is present in food that is high in salt content such as potato salad, custard, milk shakes, and
    mayonnaise. The patient presents with nausea, vomiting, and abdominal pain, followed by diarrhea beginning
    1-6 hours after ingestion of the enterotoxin.
    Vibrio cholerae(choice E) produces a secretory diarrhea due to increases in cAMP in the intestinal cells. The
    organism is not invasive. The patient presents with the sudden onset of painless, watery diarrhea that becomes
    voluminous, followed by vomiting. The stool appears nonbilious, gray, and slightly cloudy with flecks of mucus,
    no blood, and a sweet odor.

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  • 4. 

    A 37-year-old, intravenous drug-abusing male presents with fever and chills. Blood cultures are positive for Staphylococcus aureus. He develops central nervous system symptoms, and a cerebral abscess is suspected. Which part of the brain is most often affected by septic emboli in patients with infective endocarditis?

    • A.

      Brainstem

    • B.

      Cerebellum

    • C.

      Frontal lobe

    • D.

      Occipital lobe

    • E.

      Parietal lobe

    Correct Answer
    E. Parietal lobe
    Explanation
    The correct answer is E. Embolization from infective endocarditis typically causes multiple, small parietal lobe
    abscesses. This "factoid" is worth knowing because some patients with infective endocarditis present with what
    clinically looks like multiple small "strokes", and their treatable cardiac disease may be completely unsuspected.

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  • 5. 

    A 33-year-old male with AIDS and a history of shingles develops a severe, multifocal encephalitis. Therapy is instituted with acyclovir, but the man dies on the fourth day of his hospital admission. Which of the following viruses is the most likely cause of his encephalitis?

    • A.

      Cytomegalovirus

    • B.

      Herpes simplex type I

    • C.

      Herpes simplex type II

    • D.

      Herpes zoster-varicella

    • E.

      Measles virus

    Correct Answer
    D. Herpes zoster-varicella
    Explanation
    The correct answer is D. The specific clue to the cause of the severe encephalitis in this AIDS patient is the
    history of shingles, due to reactivation of the herpes zoster-varicella virus. In otherwise healthy adults, the virus
    (which is usually introduced to the body as a childhood case of chickenpox) remains dormant in a dorsal root
    ganglion, only to reactivate in later life, causing a painful vesicular eruption that characteristically conforms to
    the distribution of a single dermatome. In AIDS patients, the virus can cause a severe, multifocal encephalitis
    that may be resistant to acyclovir therapy.
    Cytomegalovirus (choice A) can cause disseminated disease (including brain infection) in AIDS patients, but is
    less likely in this patient, given the past history of shingles.
    Herpes simplex type I (choice B) and herpes simplex type II (choice C) can cause disseminated disease
    (including brain infection) in AIDS patients, but is less likely in this patient, given the past history of shingles,
    and the lack of response to acyclovir.
    Measles virus (choice E) appears to be related to subacute sclerosing panencephalitis, but this condition is not
    specifically increased in AIDS patients.

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  • 6. 

    A 35-year-old male undergoes an appendectomy. Several days later, an abscess has formed at the surgical site. It does not improve with administration of a cephalosporin, but does respond to nafcillin. The infecting organism most likely produced an enzyme that would hydrolyze which bond in the above molecule?

    • A.

      A

    • B.

      B

    • C.

      C

    • D.

      D

    Correct Answer
    D. D
    Explanation
    The correct answer is D. Abscesses are often caused by Staphylococcus aureus, an organism that may produce
    penicillinase, an enzyme that cleaves the amide bond of beta-lactam antibiotics (the molecule shown is penicillin).
    The enzyme thus confers resistance to the beta-lactam antibiotics (penicillins and cephalosporins). Nafcillin (a
    semisynthetic penicillin) is very effective against penicillinase-producing S. aureus.
    You should be familiar with other important enzymes and toxins produced by this organism, including:
    heat-resistant enterotoxin, toxic shock syndrome toxin, exfoliatin (causes scalded-skin syndrome in children), alpha toxin (kills leukocytes), and coagulase.

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  • 7. 

    A 10-year-old boy is attending summer camp in Texas. After 2 weeks of camp, he complains of a sore throat, headache, cough, and malaise. On physical examination, he also has a low-grade fever and keratoconjunctivitis. Within hours, several other campers and counselors visit the infirmary with similar symptoms. All of the patients had been swimming in the camp swimming pool. Eventually, more than 50% of the camp complain of symptoms similar to the initial case that last 5 to 7 days. Which of the following is the most likely causative organism?

    • A.

      Adenovirus

    • B.

      Chlamydia

    • C.

      Gram-negative diplococcus

    • D.

      Gram-positive enterococcus

    • E.

      Herpesvirus

    Correct Answer
    A. Adenovirus
    Explanation
    The correct answer is A. Adenoviruses are non-enveloped (naked), icosahedral DNA viruses causing a variety
    of clinical syndromes. Adenoviruses cause a pharyngoconjunctivitis that affects children and sometimes adults
    who are living in the same household. Contaminated swimming pools have been implicated as sources for the
    virus. The virus is latent in the lymphoepithelial tissue of the nasopharynx and other sites. Adenoviruses also
    cause watery, non-bloody diarrhea.
    Chlamydia spp. (choice B) produce a variety of clinical syndromes, including a sexually transmitted urethritis,
    pelvic inflammatory disease, neonatal pneumonia and inclusion conjunctivitis, lymphogranuloma venereum,
    adult interstitial pneumonia, and a zoonotic pneumonitis.
    A gram-negative diplococcus (choice C) would be a Neisseria spp. or Moraxella. Neisseria meningitidis would be
    the logical choice, since it is associated with outbreaks among children. However, the clinical syndrome caused
    by infection with the meningococcus is one of a fulminating, progressive septicemia and/or meningitis with fever,
    vascular collapse, and disseminated intravascular coagulation. N. gonorrhoeae does not cause a clinical
    syndrome as described in the question. Moraxella catarrhalis causes pneumonia in patients with chronic
    obstructive pulmonary disease.
    Gram-positive enterococcus (choice D) would refer to Group D streptococcus. These organisms are associated
    with endocarditis and genitourinary infections.
    Herpesviruses (choice E) are large, enveloped DNA viruses with an icosahedral shape. Possible diagnoses for
    this patient include Epstein-Barr virus, producing infectious mononucleosis, or cytomegalovirus, producing a
    mononucleosis-like syndrome. The patients in the question did not have hepatosplenomegaly (characteristic of
    mononucleosis), but did have keratoconjunctivitis. The organism causing the outbreak in the question also has
    a higher level of infectivity and a short incubation time.

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  • 8. 

    A 35-year-old sexually active male presents to his internist with a painless penile vesicle. Physical exam reveals inguinal lymphadenopathy. The infecting organism is definitively diagnosed and is known to exist in distinct extracellular and intracellular forms. Which of the following is the most likely pathogen?

    • A.

      Calymmatobacterium granulomatis

    • B.

      Chlamydia trachomatis

    • C.

      Haemophilus ducreyi

    • D.

      Neisseria gonorrhoeae

    • E.

      Treponema pallidum

    Correct Answer
    B. Chlamydia trachomatis
    Explanation
    The correct answer is B. This patient has lymphogranuloma venereum caused by Chlamydia trachomatis (type
    L1, 2, or 3). Chlamydia exhibit distinct infectious and reproductive forms. The extracellular infectious form is
    known as the elementary body (EB), which is incapable of reproduction. It attaches to the host cell and enters
    through endocytosis. Once inside the cell, the EB is transformed into the reticulate body (RB) within the
    endosome. The RB is capable of binary fission and divides within the endosome; fusion with other endosomes
    occurs to form a single large inclusion. Eventually, the RBs undergo DNA condensation and disulfide bond
    bridgings of the major outer membrane protein, forming EBs. The EBs are then released. Note that C.
    trachomatis is responsible for several sexually or perinatally transmitted diseases, including ocular trachoma
    (types A, B, and C), neonatal conjunctivitis, nongonococcal urethritis, cervicitis, and pelvic inflammatory disease
    (types D-K).
    Calymmatobacterium granulomatis(choice A) is a gram-negative rod that causes superficially ulcerated genital
    or inguinal papules that coalesce to form substantial lesions. The appearance of Donovan bodies in histiocytes
    is diagnostic of this infection.
    Haemophilus ducreyi(choice C) is a gram-negative rod that causes a soft, painful penile chancre, unlike that of
    a chlamydial or syphilitic lesion. This infection is common in the tropics.
    Neisseria gonorrhoeae(choice D) is a gram-negative diplococcus responsible for gonorrhea. Patients typically
    present with purulent penile discharge, not genital lesions.
    Treponema pallidum(choice E) is the spirochete responsible for syphilis. It may cause a firm, painless ulcer as a
    manifestation of primary syphilis, but the organism does not exist in distinct extracellular and intracellular forms
    as does Chlamydia. Secondary syphilis is associated with the appearance of condyloma lata–flat, gray,
    wart-like lesions.

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  • 9. 

    A 45-year-old white male with a history of alcohol abuse and periodontal disease is brought to the emergency room for a spiking fever and chills. Physical examination is significant for signs of lung consolidation. A chest x-ray shows a cavity in the right lower lobe that has an air/fluid level. A transtracheal aspiration is performed and the specimen is submitted to the laboratory for routine cultures and Gram's stain. Based upon the clinical presentation, which of the following would be the most likely finding?

    • A.

      Anaerobic bacteria

    • B.

      Aspergillus fumigatus

    • C.

      Entamoeba histolytica

    • D.

      Staphylococcus aureus

    • E.

      Streptococcus pyogenes

    Correct Answer
    A. Anaerobic bacteria
    Explanation
    The correct answer is A. Anaerobic bacteria derived from the oral flora in the clinical setting of periodontal
    disease are the most common isolates from lung abscesses. Single lung abscesses are the most common
    pattern, with the superior segment of a lower lobe or the posterior segment of an upper lobe being affected
    most often.
    Aspergillus fumigatus(choice B) presents in the lung as hemorrhagic infarctions, aspergillomas (fungus balls) in
    cavitary tuberculosis cavities, or as allergic bronchopulmonary aspergillosis.
    Entamoeba histolytica(choice C) is associated with pulmonary abscesses as an extension of a liver abscess
    across the diaphragm.
    Staphylococcus aureus(choice D) usually presents as multiple lung lesions in non-contiguous sites since the
    spread is embolic. The source of the infection is usually tricuspid endocarditis in IV drug abusers.
    Streptococcus pyogenes(choice E) typically produces a bronchopneumonia pattern following an upper
    respiratory infection.

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  • 10. 

    A traveler in Bogota, Colombia drinks a glass of fruit juice with ice cubes made from tap water. E. coli contaminating the water supply grow in the traveler's intestine and synthesize a protein that causes his intestinal epithelium to overproduce cyclic AMP, resulting in a watery diarrhea. This syndrome is typical of which of the pathogenic strains of E. coli?

    • A.

      Enteroaggregative

    • B.

      Enterohemorrhagic

    • C.

      Enteroinvasive

    • D.

      Enteropathogenic

    • E.

      Enterotoxigenic

    Correct Answer
    E. Enterotoxigenic
    Explanation
    The correct answer is E. Enterotoxigenic E. coli (ETEC), an important cause of traveler's diarrhea, produces a
    toxin that activates intestinal adenylate or guanylate cyclase. Consequently, the intestinal mucosa
    overexpresses cAMP, resulting in a mild and self-limited secretory diarrhea.
    Enteroaggregative E. coli (EAEC; choice A) does not express a toxin, but is seen to cluster over the colonic
    mucosa in some individuals. Although occasionally found in patients with chronic diarrhea, no clear mechanism
    for mucosal pathophysiology has been determined for EAEC.
    Enterohemorrhagic E. coli (EHEC; choice B), classically associated with strain O157:H7 and present in
    undercooked hamburgers, expresses a Shiga-like toxin that causes bloody diarrhea and hemolytic-uremic
    syndrome.
    Enteroinvasive E. coli (EIEC; choice C) does not express a toxin, but rather is pathogenic through its capacity to
    invade the colonic mucosa and evoke an inflammatory response. The resulting dysentery-like inflammatory
    diarrhea generally necessitates vigilant hydration, but no antibiotics are indicated.
    Enteropathogenic E. coli (EPEC; choice D) does not produce any known toxins, but adheres tightly to the
    glycocalyx of the colonic mucosa and disrupts the microvilli. Villous atrophy, mucosal thinning, and inflammation
    in the lamina propria, are produced, resulting in impaired absorption and diarrhea.

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  • 11. 

    Zygomycosis, a destructive fungal infection of the sinuses, is likely to reach the brain by which of the following routes?

    • A.

      Cavernous sinus

    • B.

      External carotid artery

    • C.

      Internal carotid artery

    • D.

      Superior sagittal sinus

    • E.

      Superior vena cava

    Correct Answer
    A. Cavernous sinus
    Explanation
    The correct answer is A. The cavernous sinuses are located on either side of the body of the sphenoid bone,
    and become a potential route of infection because they receive blood both from the face (via the ophthalmic
    veins and sphenoparietal sinus) and some of the cerebral veins. The spread of infection, especially by Mucor
    sp., into the cavernous sinus, can produce either CNS infection or cavernous sinus thrombosis, both of which
    are potentially fatal.
    The route from the face to the brain is not arterial (choices B and C).
    The superior sagittal sinus (choice D) is located in the falx cerebri, and drains venous blood from the brain to
    other dural sinuses, from which it eventually drains into the jugular vein. Zygomycosis does not reach the brain
    by way of the superior sagittal sinus.
    The superior vena cava (choice E) drains blood from the upper part of the body into the heart.

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  • 12. 

    A farmer's wife develops abdominal pain and diarrhea, followed several days later by fever, periorbital edema, eosinophilia, and myalgia. She does not remember eating anything unusual recently, but notes that she does make her own pork sausage. Which of the following techniques would be most helpful for the diagnosis of this patient?

    • A.

      Gastric biopsy

    • B.

      Muscle biopsy

    • C.

      Scotch tape test

    • D.

      Stool for ova

    • E.

      Stool for protozoal parasites

    Correct Answer
    B. Muscle biopsy
    Explanation
    The correct answer is B. The patient probably has trichinosis; the diagnosis is confirmed by demonstration of
    cysts in a muscle biopsy. Trichinosis is uncommon in this country because the organisms are usually found in
    pork and can be killed by adequate cooking. Cases in the United States are usually due to tasting of raw pork
    sausage (for seasoning purposes) or ingestion of poorly cooked bear meat.
    The organism is not usually demonstrated in stool (choices D and E) or gastric biopsies (choice A).
    The scotch tape test (choice C) is for Enterobius (pinworms).

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  • 13. 

    A 70-year-old man with a history of prostate cancer presents with a chief complaint of pain on the right side of his chest for the past several days. He has been receiving external beam radiation to spinal metastases of his prostate cancer for the past several weeks. On examination, there is marked tenderness along the right side of the chest wall in a 4-6 cm stripe from the midline to the flank. Multiple small vesicular lesions are visible in this area on an erythematous base. Some of the lesions are fluid-filled, and some are crusted. How do members of the virus family responsible for his condition produce messenger RNA?

    • A.

      By direct translation from the genome

    • B.

      By producing a double-stranded DNA intermediate

    • C.

      By producing a negative sense intermediate

    • D.

      By producing a positive sense intermediate

    • E.

      By transcribing the genomic DNA

    • F.

      By transcription from proviral DNA

    • G.

      The genomic RNA is used directly on the ribosome

    Correct Answer
    E. By transcribing the genomic DNA
    Explanation
    The correct answer is E. This is a classic case of shingles, a stress-activated secondary disease caused by
    reactivation herpes zoster. Herpesviruses are DNA viruses that use the mechanisms used by our own cells to
    transcribe an RNA strand from their genomic DNA and use the transcribed RNA as messenger RNA.
    Positive RNA viruses use direct translation from the genome (choice A) to make protein, not messenger RNA.
    Retroviruses produce a double-stranded DNA intermediate (choice B) to effect genomic duplication.
    Positive sense RNA viruses produce a negative sense intermediate (choice C) to produce a genomic duplicate.
    Negative sense RNA viruses produce a positive sense intermediate (choice D) to produce messenger RNA and
    a genomic duplicate.
    Transcription from proviral DNA (choice F) is used by the retroviruses to make messenger RNA.
    Genomic RNA used directly on the ribosomes (choice G) characterizes the production of messenger RNA by
    positive sense RNA viruses.

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  • 14. 

    A 66-year-old man with urinary retention secondary to prostatic hyperplasia develops a spiking fever and tachypnea. Physical exam reveals intercostal muscle retractions and bilateral inspiratory crackles. A chest x-ray exhibits bilateral interstitial and alveolar infiltrates. Arterial blood gases demonstrate severe hypoxemia. Blood cultures would most likely reveal

    • A.

      Gram-negative diplococci

    • B.

      Gram-negative rods

    • C.

      Gram-positive cocci

    • D.

      Gram-positive diplococci

    • E.

      Gram-positive rods

    Correct Answer
    B. Gram-negative rods
    Explanation
    The correct answer is B.Escherichia coli is a very common pathogen associated with urinary tract infections and
    is a common cause of cystitis. It is part of the normal flora of the GI tract. Patients with cystitis can develop
    bacteremia and subsequent septic shock and adult respiratory distress syndrome (ARDS). The patient
    described in the history has an obstructive lesion of the urinary tract that predisposes him to the overgrowth of
    microorganisms such as E. coli. Pulmonary symptoms are consistent with ARDS. E. coli is a gram-negative rod.
    Gram-negative diplococci (choice A) might be Neisseria spp. or Moraxella catarrhalis. Neisseria gonorrhea is
    the cause of gonorrhea, a sexually transmitted disease that presents with urethritis, or may be asymptomatic.
    Patients are usually younger, sexually active males. Moraxella spp. and Kingella kingae are gram-negative
    cocci that can cause a wide variety of infections. Moraxella catarrhalis is usually implicated as a cause of otitis
    media and sinusitis in children, or as a cause of purulent tracheobronchitis and pneumonia in a population of
    patients who are over 50 and have underlying obstructive lung disease.
    Gram-positive cocci (choice C), such as Staphylococcus aureus and S. epidermidis and Streptococci, rarely
    cause cystitis. Staphylococcus saprophyticus causes urinary tract infections, but the patients are typically
    young, sexually active women.
    Gram-positive diplococci (choice D) would be a description of Streptococcus pneumoniae, which is the most
    common cause of community-acquired pneumonia.
    Gram-positive rods (choice E) would include members of the following genera: Clostridium, Bacillus, Listeria,
    and the coryneform bacteria. The only significant member of this group to produce urinary tract infections is Corynebacterium urealyticum. (C. jeikeium). The organism creates an alkaline urine environment with the potential for stone formation. Patients are usually immunocompromised or have had recurrent urinary tract infections.

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  • 15. 

    A sexually active 25-year-old man develops epididymitis and orchitis. Needle biopsy demonstrates a prominent leukocytic infiltrate with numerous neutrophils. Which of the following organisms is the most likely cause of this man's infection?

    • A.

      Escherichia coli

    • B.

      Mycobacterium tuberculosis

    • C.

      Neisseria gonorrhoeae

    • D.

      Pseudomonas sp.

    • E.

      Treponema pallidum

    Correct Answer
    C. Neisseria gonorrhoeae
    Explanation
    The correct answer is C. Acute epididymitis and orchitis with prominent neutrophils in a sexually active male are
    most likely due to infection with Neisseria gonorrhoeae or Chlamydia trachomatis. N. gonorrhoeae can produce
    a nonspecific pattern of acute inflammation (nonspecific epididymitis and orchitis) or can be sufficiently severe
    as to cause frank abscesses within the epididymis.
    Escherichia coli(choice A) is an important cause of nonspecific epididymitis and orchitis in children with
    congenital genitourinary abnormalities and in older men.
    Mycobacterium tuberculosis(choice B) can cause tuberculosis of the epididymis and testes, characterized by
    granuloma formation.
    Pseudomonas sp.(choice D) has been implicated as an important cause of nonspecific epididymitis and orchitis
    in older men.
    Treponema pallidum(choice E), the causative agent of syphilis, can cause testicular involvement with gumma
    formation, endarteritis, and/or a prominent plasma cell infiltrate.

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  • 16. 

    Which of the following organisms is the most common cause of community-acquired pneumonia?

    • A.

      Chlamydia pneumoniae

    • B.

      Haemophilus influenzae

    • C.

      Mycoplasma pneumoniae

    • D.

      Staphylococcus aureus

    • E.

      Streptococcus pneumoniae

    Correct Answer
    E. Streptococcus pneumoniae
    Explanation
    The correct answer is E. The most common bacteria implicated in community-acquired pneumonia is the
    pneumococcus, Streptococcus pneumoniae. Other organisms frequently implicated in patients less than age 60
    without comorbidity include Mycoplasma pneumoniae, respiratory viruses, Chlamydia pneumoniae, and
    Haemophilus influenzae. When community-acquired pneumonia occurs in elderly patients or patients with
    comorbidity, aerobic gram-negative bacilli and Staphylococcus aureus are added to the list.
    The organisms listed in choices A, B, and C are important causes of community-acquired pneumonia, but are
    not the most frequent causes.
    Staphylococcus aureus(choice D) is an important cause of community-acquired pneumonia (particularly in the
    elderly and in patients with comorbidity), but is not the most frequent cause.

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  • 17. 

    A patient with a history of chronic diarrhea and bloody stools presents to the emergency department with right upper quadrant pain and fever. Physical examination demonstrates a large, tender liver. Which of the following would be most likely to have caused the patient's problems?

    • A.

      Cryptosporidium parvum

    • B.

      Entamoeba histolytica

    • C.

      Giardia lamblia

    • D.

      Isospora belli

    • E.

      Trichomonas vaginalis

    Correct Answer
    B. Entamoeba histolytica
    Explanation
    The correct answer is B.Entamoeba histolytica is an intestinal amoebic parasite that either can cause relatively
    mild diarrhea, or can behave more aggressively, causing dysentery (bloody diarrhea with abdominal pain and
    dehydration), peritonitis, or liver abscess formation (such as this patient has). Treatment is with metronidazole.
    Cryptosporidium parvum(choice A), Giardia lamblia(choice C), and Isospora belli(choice D) can cause chronic
    diarrhea, but would not be expected to cause a liver abscess.
    Trichomonas vaginalis(choice E) is not a cause of diarrhea, but instead causes vaginitis.

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  • 18. 

    A neonate is born in very poor condition, with a severe, generalized encephalitis. Which of the following viruses is the most likely pathogen in this setting?

    • A.

      Eastern equine encephalitis virus

    • B.

      Herpes simplex type II

    • C.

      Herpes zoster-varicella virus

    • D.

      Poliomyelitis virus

    • E.

      St. Louis encephalitis virus

    Correct Answer
    B. Herpes simplex type II
    Explanation
    The correct answer is B. Viral causes of neonatal encephalitis include three members of the herpes family of
    viruses: herpes simplex I, herpes simplex II, and cytomegalovirus. All three types can have devastating effects
    on the neonate, with extensive CNS damage leading to mental retardation, seizures, and focal neurologic
    problems. Acyclovir may be of some help in modifying these infections, but both treatment and prognosis
    remain very problematic.
    Eastern equine encephalitis virus (choice A) and St. Louis encephalitis virus (choice E) are causes of epidemic
    encephalitis but are not the most likely cause of neonatal encephalitis.
    Herpes zoster-varicella virus (choice C), unlike herpes simplex, is not usually a cause of neonatal encephalitis.
    Poliomyelitis virus (choice D) is a gastrointestinally transmitted virus that is not usually encountered in neonates
    (or anyone else in the U.S. currently).

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  • 19. 

    A 15-year-old high school student and several of her friends ate lunch at a local Chinese restaurant. They all were served the daily luncheon special, which consisted of sweet and sour pork with vegetables and fried rice. All of the girls developed nausea, vomiting, abdominal pain, and diarrhea within 6 hours of eating lunch. Which of the following is the most likely cause of these symptoms?

    • A.

      Bacillus cereus

    • B.

      Clostridium botulinum

    • C.

      Clostridium perfringens

    • D.

      EHEC (Enterohemorrhagic E. coli)

    • E.

      Staphylococcus aureus

    • F.

      Vibrio cholerae

    Correct Answer
    A. Bacillus cereus
    Explanation
    The correct answer is A.Bacillus cereus produces a self-limited diarrhea due to ingestion of the preformed
    enterotoxin in contaminated fried rice and seafood. The incubation period is typically around 4 hours. The
    degree of vomiting is greater than the diarrhea. B. cereus is also associated with keratitis, producing a corneal
    ring abscess.
    Clostridium botulinum(choice B) produces a neurotoxin that blocks the release of acetylcholine, resulting in a symmetric descending paralysis that may lead to respiratory complications causing death. Symptoms include
    blurred vision, photophobia, dysphagia, nausea, vomiting, and dysphonia. Most cases are associated with the
    ingestion of contaminated home-canned food.
    Clostridium perfringens(choice C) produces a severe diarrhea with abdominal pain and cramping (sometimes
    called "church picnic" diarrhea). The incubation period is 8-24 hours after ingesting contaminated meat, meat
    products, or poultry. The meats have usually been cooked, allowed to cool, and then warmed, which causes
    germination of the clostridial spores.
    EHEC-Enterohemorrhagic E. coli(choice D), produces a bloody, non-invasive diarrhea due to the ingestion of
    verotoxin found in undercooked hamburger at fast food restaurants. The 0157:H7 serotype typically produces
    this syndrome. Some patients develop a life-threatening complication called hemolytic-uremic syndrome.
    Staphylococcus aureus(choice E) produces a self-limited food poisoning syndrome with nausea, vomiting, and
    abdominal pain followed by diarrhea beginning 1-6 hours after ingestion of the enterotoxin. The organism is
    found in foods such as potato salad, custard, milk shakes, and mayonnaise.
    Vibrio cholerae(choice F) typically produces a watery, non-bloody diarrhea with flecks of mucus (rice-water
    stools). Abdominal pain is not a feature. Massive fluid loss and electrolyte imbalance are complications. In the
    U.S., cases of cholera (El Tor 01 strain) are associated with the Gulf coast and ingestion of poorly cooked or
    poorly stored crabs, shrimp, or oysters. A strain of V. cholerae called non-01 is also found along the Gulf coast.
    Patients who ingest contaminated shellfish experience fever, copious watery diarrhea, and abdominal cramps
    within 48 hours after eating.

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  • 20. 

    A 29-year-old news correspondent returns from covering an earthquake and its aftermath in a third world country. The man feels tired and has sore muscles, so he consults a physician, who examines him, but decides not to admit him to the hospital. The man subsequently develops a disabling illness complicated by severe anemia, pulmonary edema, renal failure, and shock. Which of the following organisms is the most likely pathogen?

    • A.

      Babesia microti

    • B.

      Plasmodium falciparum

    • C.

      Plasmodium malariae

    • D.

      Plasmodium ovale

    • E.

      Plasmodium vivax

    Correct Answer
    B. Plasmodium falciparum
    Explanation
    The correct answer is B. Most forms of malaria cause chronic disease with significant morbidity but very little
    mortality. The exception is malaria caused by Plasmodium falciparum (sometimes still called malignant tertian
    malaria), which has a propensity for being severe and having life-threatening complications. Falciparum malaria
    may have very severe anemia with hemoglobin less than 5 g/dL and more than half of the erythrocytes bearing
    parasites. Complications of this form of malaria include pulmonary edema, renal failure with or without
    hemoglobinuria, shock, hypoglycemia, and cerebral malaria. Episodes occur with an irregular periodicity with
    fever spikes at 48-hour intervals during symptomatic periods.
    Babesia microti(choice A) is an intraerythrocytic parasite that causes relatively mild, self-limited disease called
    babesiosis.
    Plasmodium malariae(choice C) causes relatively mild malaria characterized by fever spikes with a 72-hour
    periodicity.
    Plasmodium ovale(choice D) and Plasmodium vivax(choice E) cause relatively mild malaria characterized by
    fever spikes with a 48-hour periodicity.

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  • 21. 

    A 9-month-old infant is brought to the Health Department to receive the second dose of OPV (oral polio vaccine) 2 weeks after the first vaccination. The child has mild diarrhea, so the decision is made to defer further immunizations. Bacteriologic examination of a stool culture is unremarkable; however, a small, single-stranded, positive RNA virus is isolated from the specimen. This same agent was isolated from sewage effluent the preceding week. The viral isolate was not inactivated by ether. Which of the following viruses was most likely isolated?

    • A.

      Adenovirus

    • B.

      Hepatitis C

    • C.

      Parvovirus B19

    • D.

      Poliovirus

    • E.

      Rotavirus

    Correct Answer
    D. Poliovirus
    Explanation
    The correct answer is D. Poliovirus, which is a single-stranded +RNA virus, is naked (i.e., non-enveloped) and
    hence will not be inactivated by lipid solvents such as ether. The live virus vaccine had colonized the intestinal
    tract of the infant and was still being shed 2 weeks after the earlier oral dose. This same virus, the vaccine
    strain, is likely to be found in sewage, as all vaccinated infants will shed virus for a period of time after
    immunization with OPV.
    Adenoviruses (choice A) and parvovirus B19 (choice C) also may cause diarrheal disease and both are
    non-enveloped; however, they both have a DNA genome.
    Hepatitis C (choice B) is an enveloped, single-stranded +RNA virus; its major target organ is the liver, not the
    intestinal tract. It is a fragile agent that does not survive well outside the body and would not be isolated from
    raw sewage effluent.
    Rotavirus (choice E) is the major cause of diarrheal disease in infants under the age of 2 years. It is a member
    of the reovirus family and, as such, is double-stranded. This virus causes hospitalization of 30% to 40% of the
    infected infants and kills hundreds of thousands of infants in developing nations where access to hospitals is
    not readily available. Therapy for the watery diarrhea produced by this agent is fluid and electrolyte
    replacement.

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  • 22. 

    A 24-year-old male Asian immigrant presents with an ulcerative genital lesion. The lesion first appeared 1 month ago as a papule with an erythematous base, which eventually became ulcerated and painful. On physical examination, the man is afebrile. A tender ulcerative lesion is present on his prepuce, and inguinal adenopathy is evident. Which of the following would be the most likely microscopic finding in a scraping from the rash?

    • A.

      Epithelial cells with intranuclear inclusion bodies

    • B.

      Iodine-staining intracellular inclusion bodies

    • C.

      Koilocytotic squamous epithelial cells

    • D.

      Neutrophils containing gram-negative diplococci

    • E.

      Pleomorphic gram-negative rods in a "school of fish" pattern

    • F.

      Spirochetes visible by darkfield microscopy

    Correct Answer
    E. Pleomorphic gram-negative rods in a "school of fish" pattern
    Explanation
    The correct answer is E. This is a typical case description of chancroid, caused by Haemophilus ducreyi, a
    pleomorphic gram-negative rod that displays a characteristic pattern on Gram's stained slides.
    Epithelial cells with intranuclear inclusion bodies (choice A) would be found with herpes simplex infections, but
    these lesions would not have the appearance described in this case history.
    Iodine-staining intracellular inclusion bodies (choice B) would be found with genital lesions of Chlamydia
    trachomatis, the causative agent of lymphogranuloma venereum, but this lesion is generally nonpainful.
    Koilocytotic squamous epithelial cells (choice C) would be found in infections with human papilloma virus (HPV),
    which is associated with venereal warts.
    Neutrophils containing gram-negative diplococci (choice D) would be found if this were gonorrhea, but the case
    symptoms are not consistent with this disease.
    Spirochetes (choice F) would be found on darkfield microscopy if this were a case of syphilitic chancre, but that
    chancre would be hard and nontender.

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  • 23. 

    A burn patient at the university hospital has been progressively deteriorating. He was catheterized for several days and developed a severe pneumonia, for which he was intubated and is now ventilator-dependent. A gram-negative, non-fermenting rod is isolated from his sputum. It produces a blue-green pigment on growth media and has a grape-like fruity odor. The organism most likely isolated is

    • A.

      Escherichia coli

    • B.

      Klebsiella pneumoniae

    • C.

      Legionella pneumophila

    • D.

      Pseudomonas aeruginosa

    • E.

      Serratia marcescens

    Correct Answer
    D. Pseudomonas aeruginosa
    Explanation
    The correct answer is D. Pseudomonas aeruginosa is a very common opportunist in burn patients, in whom it
    classically causes secondary wound infections and septicemia. It may also cause cystitis in patients with urinary
    catheters and pneumonia in patients with cystic fibrosis. The organism is found in water and usually gains
    access to the body as a contaminant in the water used in respirators or in water baths used to cleanse wounds.
    This organism is a non-fermenter, that is, it does not metabolize sugars by classic pathways. It produces a
    blue-green, water-soluble pigment (pyocyanin), and has a fruity odor when grown on laboratory media.
    Escherichia coli(choice A) is a lactose-fermenting, gram-negative rod commonly seen as normal flora of the
    intestine of man. It is the most common cause of urinary bladder infections, pyelonephritis, and sepsis in
    patients with indwelling urinary catheters. It is also the major cause of traveler's diarrhea and is a very important
    pathogen in neonates who become infected during passage through the birth canal.
    Klebsiella pneumoniae(choice B) is a gram-negative, highly encapsulated rod that is a significant pulmonary
    pathogen in individuals with a respiratory compromise. It is a common cause of aspiration pneumonia and
    pulmonary abscesses in alcoholics and patients with chronic obstructive pulmonary disease. The organism is
    readily grown on standard laboratory media such as blood agar or MacConkey's enteric agar.
    Legionella pneumophila(choice C) is a fastidious, gram- negative respiratory pathogen that may cause either a
    fulminating disease or a mild "walking pneumonia-like" condition (i.e., an atypical pneumonia). The organism
    can be cultured on a charcoal yeast extract medium, but identification is usually accomplished by
    immunofluorescent examination of the pulmonary specimen.
    Serratia marcescens(choice E) is a gram-negative organism that is found in water, soil, and as an occasional normal flora of humans. It is an opportunistic pathogen that causes respiratory disease in hospitalized patients. Many strains produce a pigment, but the colonies are usually red, pink, or orange. Interestingly, the pathogenic varieties are most often non-pigmented.

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  • 24. 

    A 57-year-old fisherman with a history of alcoholism is hospitalized in Gulfport, Mississippi with a 1-day history of severe, watery diarrhea after eating several raw oysters. He is badly dehydrated on admission, and within 12 hours, he becomes severely hypotensive and dies. Which of the following pathogens is the most likely cause of this man's death?

    • A.

      Citrobacter diversus

    • B.

      Enterotoxigenic E. coli

    • C.

      Providencia stuartii

    • D.

      Vibrio cholerae

    • E.

      Vibrio vulnificus

    Correct Answer
    E. Vibrio vulnificus
    Explanation
    The correct answer is E.Vibrio vulnificus is an extremely invasive organism, producing a septicemia in patients
    after eating raw shellfish, or causing wound infections, cellulitis, fasciitis, and myositis after exposure to
    seawater or after cleaning shellfish. Patients at high risk for septicemia include those with liver disease,
    congestive heart failure, diabetes mellitus, renal failure, hemochromatosis, and immunosuppression.
    Citrobacter diversus(choice A) produces neonatal meningitis and can be frequently cultured from the umbilicus.
    Enterotoxigenic E. coli(choice B) produces the classic traveler's diarrhea. The toxin is ingested in water and
    salads. The incubation period is approximately 12 hours. The diarrhea is non-inflammatory and treatment is
    supportive.
    Providencia stuartii(choice C) is a gram-negative rod related to Proteus. It is a common cause of nosocomial
    bacteremia in nursing home patients with chronic catheterization.
    Vibrio cholerae(choice D) produces a non-invasive, non-inflammatory, high-volume secretory diarrhea that is
    toxin-mediated.

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  • 25. 

    A 38-year-old AIDS patient presents to his physician's office in Kansas City, Missouri, complaining of fever for the past week and an increasing headache. He also states that sunlight hurts his eyes and that he has been feeling nauseated and weak. His past medical history is significant for Pneumocystis pneumonia and a total CD4 count of 89. Current medications are trimethoprim/sulfa and indinavir. Cerebrospinal fluid (CSF) reveals 4 WBC, and budding encapsulated yeast forms grow on Sabouraud's agar. Which of the following is an accurate description of the morphology of the infectious form of the organism responsible for the man's illness?

    • A.

      Broad-based, budding yeasts

    • B.

      Budding yeasts in a "pilot's wheel" arrangement

    • C.

      Cylindrical arthroconidia

    • D.

      Encapsulated budding yeasts

    • E.

      Filamentous molds

    • F.

      Septate hyphae with microconidia and macroconidia

    Correct Answer
    D. Encapsulated budding yeasts
    Explanation
    The correct answer is D. This is a classic case of Cryptococcus neoformans meningitis. Clues included the
    patient population (HIV positive), geographic area (Mississippi and Missouri river beds), and diagnostic form
    (encapsulated yeast). Cryptococcus is a monomorphic fungus, unlike many of the classic pathogens within the
    fungal group, so the encapsulated yeast form would be found both in clinical specimens and in the environment
    as the infectious form.
    Broad-based, budding yeasts (choice A) would be the forms expected to be found in clinical specimens (not
    environmental forms) from patients infected with Blastomyces dermatitidis, which is far more likely to present
    with skin and bone lesions than with meningitis.
    Budding yeasts in a "pilot's wheel" arrangement (choice B) would be the form expected to be found in clinical
    specimens (not environmental forms) from patients infected with Paracoccidioides brasiliensis, which is limited in
    geographic region to Central and South America and typically presents as a primary pulmonary disease.
    Cylindrical arthroconidia (choice C) would be the transmission forms characteristic of Coccidioides immitis,
    which may be a cause of fungal meningitis in immunologically compromised individuals, but is geographically
    restricted to the sub-Sonoran desert zone of the U.S. (San Joaquin valley fever).
    Filamentous molds (choice E) are the transmission forms of several of the fungal agents (eg, Coccidioides,
    Blastomyces, Paracoccidioides) but Cryptococcus does not have a filamentous form.
    Septate hyphae with microconidia and macroconidia (choice F) are the transmission (environmental) forms of Histoplasma capsulatum, which is not an encapsulated yeast in spite of its name. It is primarily a pulmonary infection acquired by exposure to the droppings of birds or bats.

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