USMLE : Microbiology Toughest Test! Trivia Quiz

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USMLE : Microbiology Toughest Test! Trivia Quiz - Quiz


Microbiology is the study of organisms, those being unicellular or acellular. Microbiology involves numerous sub-disciplines, including bacteriology, virology, immunology, and physiology. It is the branch of biology that studies micro- organisms. The study of the human immune system is an example of microbiology. If you are fascinated by microbiology and want to challenge yourself, try taking this test.


Questions and Answers
  • 1. 

    A baby born at 32 weeks gestation with Apgar scores of 2 and 7 was placed in the neonatal intensive care unit. She developed a fever and leukocytosis; lumbar puncture revealed pleocytosis with increased protein, decreased glucose, and gram-positive rods. Which one of the following organisms was most likely isolated from the CSF?

    • A.

      Escherichia coli

    • B.

      Listeria monocytogenes

    • C.

      Neisseria meningitidis

    • D.

      Streptococcus agalactiae

    • E.

      Streptococcus pneumoniae

    Correct Answer
    B. Listeria monocytogenes
    Explanation
    The correct answer is B. The three major causes of neonatal meningitis are group B streptococci
    (Streptococcus agalactiae; choice D), Escherichia coli(choice A), and Listeria monocytogenes. All can be found
    in the vaginal tract of normal women and may contaminate the infant during passage through the birth canal.
    They colonize the upper respiratory tract and can cause pneumonia, septicemia, and/or meningitis in the
    neonate. They are readily distinguished on morphologic grounds; the streptococci are gram-positive cocci in
    chains, E. coli is a gram-negative rod, and L. monocytogenes is a gram-positive pleomorphic rod. There are
    other gram-positive rods that resemble Listeria (e.g., the diphtheroid bacilli found in the upper respiratory tract
    and on the skin), hence a motility test is done to confirm the identification. L. monocytogenes is motile at room
    temperature but not when grown at 37 degrees C. L. monocytogenes is also associated with drinking
    unpasteurized milk.
    Neisseria meningitidis(choice C) is the most common cause of meningitis in school-age children and young
    adults. It is a fastidious, non-motile, gram-negative diplococcus that would be a very rare cause of meningeal
    disease in very young patients, such as this one.
    Streptococcus pneumoniae(choice E) is a gram-positive coccus that grows in pairs and short chains. It is the
    number one cause of pneumonia, septicemia, and meningitis in the elderly. There is a vaccine composed of the
    capsular carbohydrate of 23 serotypes of this organism that 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 the
    development of pneumococcal sepsis.

    Rate this question:

  • 2. 

    A 27-year-old intravenous drug user presents with difficulty swallowing. Examination of the oropharynx reveals white plaques along the tongue and the oral mucosa. Which of the following best describes the microscopic appearance of the microorganism responsible for this patient's illness?

    • A.

      Budding yeast and pseudohyphae

    • B.

      Encapsulated yeast

    • C.

      Mold with nonseptate hyphae

    • D.

      Mold with septate hyphae

    Correct Answer
    A. Budding yeast and pseudohyphae
    Explanation
    The correct answer is A. This patient has Candida esophagitis. Any time a patient presents with dysphagia or
    odontophagia and you note white plaques in the oropharnyx (thrush), you can assume that the Candida is
    affecting the esophagus as well. The fact that the patient is an IV drug user makes an opportunistic infection
    such as Candida more likely. Now comes the hard part–remembering the morphology of Candida.
    Candida appears as budding yeast with pseudohyphae in vivo.
    The other answer choices represent the morphology of other important opportunistic fungi:
    Cryptococcus are encapsulated yeast (choice B). You should think about Cryptococcus neoformans when
    you're presented with an immunocompromised patient with neurological symptoms. The classic clue is the
    presence of encapsulated organisms observable in an India ink preparation.
    Mucor and Rhizopus are molds with nonseptate hyphae (choice C). You should think about Mucor when you are
    presented with a diabetic (especially ketoacidosis) or a leukemic patient with a severe sinus infection.
    Aspergillus is a mold with septate hyphae (choice D). In immunocompromised patients, aspergillosis can present
    with acute pneumonia, often with cavitation (aspergillomas = fungus balls in the lungs).

    Rate this question:

  • 3. 

    A 24-year-old woman in her third trimester of pregnancy presents with urinary frequency and burning for the past few days. She denies fever, nausea, vomiting, or chills. She takes no medications besides prenatal vitamins and is generally in good health. Physical examination is remarkable for mild suprapubic tenderness, and a urine dipstick is positive for white blood cells, protein, and a small amount of blood. Culture produces greater than 100,000 colonies of gram-negative bacilli. Which of the following attributes of this uropathogenic organism is most strongly associated with its virulence?

    • A.

      Bundle-forming pili

    • B.

      GVVPQ fimbriae

    • C.

      Heat labile toxins

    • D.

      Heat stable toxins

    • E.

      P pili

    • F.

      Type 1 pili

    Correct Answer
    E. P pili
    Explanation
    The correct answer is E. Urinary tract infections are the most common bacterial infections encountered during
    pregnancy, and Escherichia coli is the most commonly isolated organism. 70% of cases in the U.S. are caused
    by P pili-positive strains.
    Bundle-forming pili (choice A) are found in enteroaggregative E. coli (EAEC).
    GVVPQ fimbriae (choice B) are found in EAEC.
    Heat labile toxins (choice C) are pathogenic factors in enterotoxic strains (ETEC).
    Heat stable toxins (choice D) are pathogenic factors in ETEC or EAEC.
    Type 1 pili (choice F) are a major pathogenic factor in ETEC.

    Rate this question:

  • 4. 

    A febrile 23-year-old college coed presents with fatigue and difficulty swallowing. Physical exam reveals exudative tonsillitis, palatal petechiae, cervical lymphadenopathy, and tender hepatosplenomegaly. A complete blood count reveals mild anemia, lymphocytosis with about 30% of the lymphocytes exhibiting atypical features, and a mild thrombocytopenia. Coombs' test is positive. Which of the following is the most likely complication of this syndrome?

    • A.

      Acute cholecystitis

    • B.

      Ascending cholangitis

    • C.

      Diarrhea

    • D.

      Immune complex vasculitis

    • E.

      Splenic rupture

    Correct Answer
    E. Splenic rupture
    Explanation
    The correct answer is E. The syndrome represented by the clinical vignette is infectious mononucleosis.
    Epstein-Barr virus (EBV) is the usual cause of heterophile-positive infectious mononucleosis; cytomegalovirus responsible for a minority of cases. Rarely, splenic rupture requiring splenectomy can result from splenomegaly
    and capsular swelling, usually occurring during the 2nd and 3rd weeks of the illness.
    Acute cholecystitis (choice A) is not associated with infectious mononucleosis. The most frequently isolated
    pathogens are E. coli, Klebsiella spp., group D Streptococcus, Staphylococcus spp., and Clostridium spp.
    Ascending cholangitis (choice B) is not associated with infectious mononucleosis. Cholangitis usually presents
    with biliary colic, jaundice, and spiking fever with chills (Charcot's triad). Blood cultures are usually positive (E.
    coli is a common isolate), with an accompanying leukocytosis. AIDS-related cholangitis has been reported,
    presenting with abdominal pain and obstructive liver symptoms. Potential etiologic agents include
    Cytomegalovirus, Cryptosporidium parvum, and Microsporidia, including Enterocytozoon cuniculi.
    Diarrhea (choice C) is not usually produced by infectious mononucleosis.
    Immune complex vasculitis (choice D) is not associated with infectious mononucleosis.

    Rate this question:

  • 5. 

    A 37-year-old newly married man presents with multiple blister-like lesions on the glans of his penis, appearing over the past 2 days. On questioning, he recalls similar episodes over the past 2 years. Examination is remarkable for tender, 3-4 mm vesicular lesions on the shaft of his penis with no apparent crusting, drainage, or bleeding. There is also slight bilateral inguinal adenopathy. During the asymptomatic period between outbreaks, where would the causative agent likely have been found?

    • A.

      Fibroblasts

    • B.

      Lymphocytes

    • C.

      Macrophages

    • D.

      Mucoepithelial cells

    • E.

      Neurons of the sacral ganglia

    • F.

      Neurons of the trigeminal ganglia

    Correct Answer
    E. Neurons of the sacral ganglia
    Explanation
    The correct answer is E. This is a classic example of an infection with herpes simplex virus (probably type 2).
    This agent causes lytic infections in mucoepithelial cells. It is then retrogradely transported into neurons of the
    sacral ganglia, where it lays dormant during the asymptomatic phase of the disease.
    Fibroblasts (choice A) may be infected by cytomegalovirus, another herpesvirus, but this produces a distinctive
    mononucleosis-like syndrome in most normal individuals.
    Lymphocytes (choice B) and macrophages (choice C) may be infected by herpes simplex type 2 but are not the
    site of residence of the virus during quiescent periods.
    Infection of mucoepithelial cells (choice D) by herpes simplex produces vesicular-like lesions on the mucous
    membranes during symptomatic periods.
    Neurons of the trigeminal ganglia (choice F) may be latently infected with herpes simplex type 1. This agent is a
    possible cause of genital ulcers, but is usually associated with perioral lesions (cold sores).

    Rate this question:

  • 6. 

    A 67-year-old woman in a skilled nursing facility complains of flu-like symptoms. After several days, she develops high fever, dyspnea, and a productive cough. The nurses also notice mental status changes, and she is transported to the nearby community hospital. A chest x-ray shows a cavitary lesion in her left lung. Which of the following organisms would most likely be identified from examination of her sputum?

    • A.

      Candida albicans

    • B.

      Klebsiella pneumoniae

    • C.

      Pneumocystis carinii

    • D.

      Staphylococcus aureus

    • E.

      Streptococcus pneumoniae

    Correct Answer
    D. Staphylococcus aureus
    Explanation
    The correct answer is D. The woman has developed a pulmonary abscess, as evidenced by the chest
    radiograph. Of the organisms listed, Staph. aureus is the most likely cause of bacterial pneumonia complicated
    by abscess formation. Bacteria commonly producing pneumonia developing in the context of influenza include
    Staphylococcus aureus, Haemophilus influenzae, and Streptococcus pneumoniae, but Streptococcus
    pneumoniae is not a frequent cause of lung abscesses.
    Candida albicans(choice A) would not be a likely cause of this woman's abscess, or of the pneumonia that
    preceded it.
    Klebsiella pneumoniae(choice B) is more likely in men who are alcoholic or diabetic, or suffering from chronic
    obstructive lung disease.
    Pneumocystis carinii(choice C) is associated with pneumonia in immunocompromised hosts, but there is no
    indication of immune compromise in this patient.
    Streptococcus pneumoniae(choice E) is only rarely associated with lung abscess.

    Rate this question:

  • 7. 

    A man presents to a dermatologist because of a severe mucocutaneous rash that involves most of his body, including his palms and soles. Questioning reveals that he is a merchant marine who several months previously had an encounter with a prostitute in Southeast Asia. Which of the following is the most likely causative agent of this rash?

    • A.

      Herpes simplex I

    • B.

      Herpes simplex II

    • C.

      HIV

    • D.

      Neisseria gonorrhoeae

    • E.

      Treponema pallidum

    Correct Answer
    E. Treponema pallidum
    Explanation
    The correct answer is E. The rash described is that of secondary syphilis, caused by Treponema pallidum.
    Involvement of palms and soles by a rash is unusual, and secondary syphilis should come to mind. Not all
    patients with secondary syphilis have a severe form of the rash, and consequentially some cases are missed.
    Primary syphilis takes the form of a painless, button-like mass called chancres. Tertiary syphilis, which is now
    rare, has a propensity for involving the aorta and central nervous system and can also cause "gummas"
    (granulomatous-like lesions) in many sites, notably including liver and bone. Herpes simplex I (choice A) usually causes perioral vesicular lesions.
    Herpes simplex II (choice B) usually causes genital vesicular lesions.
    HIV (choice C) does not itself cause a rash, although co-infection with other organisms can result in a rash. Neisseria gonorrhoeae(choice D) does not typically cause a rash.

    Rate this question:

  • 8. 

    A child is brought to the pediatrician because of perianal itching, which has been disturbing the child's sleep. Physical examination reveals scaly skin in the perianal region. The physician instructs the parents to place transparent tape on the perianal region the next morning, and then return to the office. In the office, the tape is transferred to a microscope slide, and low-power examination of the tape reveals oval eggs that are flattened along one side. The microorganism most likely responsible for the child's symptoms is

    • A.

      Ancylostoma duodenale

    • B.

      Ascaris lumbricoides

    • C.

      Enterobius vermicularis

    • D.

      Necator americanus

    • E.

      Trichuris trichiura

    Correct Answer
    C. Enterobius vermicularis
    Explanation
    The correct answer is C.Enterobius vermicularis (pinworm) inhabits the large intestine, and the gravid females
    migrate at night into the perianal region to lay eggs, which can be identified by placing cellophane tape on the
    perianal skin and then looking at the tape with a microscope. Enterobiasis is characterized by extreme pruritus
    and is very contagious, transmitted by ingestion of the eggs of the organism. The infection is easily treated with
    a single dose of mebendazole.
    Ancylostoma duodenale(choice A) and Necator americanus(choice D) are hookworms, and typically cause
    anemia.
    Ascaris lumbricoides(choice B) is a large, intestinal roundworm that is very common, especially in the tropics.
    Infection may be asymptomatic or can cause abdominal symptoms.
    Trichuris trichiura(choice E) is the whipworm, which can cause rectal prolapse with heavy infestations.

    Rate this question:

  • 9. 

    One week following a visit to the woods along an Eastern seaboard beach, a 50-year-old woman develops fever, headache, chills, and fatigue. A blood smear demonstrates protozoa within erythrocytes. Which of the following is the most likely pathogen?

    • A.

      Babesia microti

    • B.

      Leishmania donovania

    • C.

      Plasmodium falciparum

    • D.

      Plasmodium vivax

    • E.

      Trypanosoma cruzi

    Correct Answer
    A. Babesia microti
    Explanation
    The correct answer is A. The combination of a one week incubation period, the Eastern seaboard clue, and the
    intra-erythrocyte parasites strongly suggest Babesia microti as the infecting organism. The clinical disease is
    called babesiosis. The infecting protozoan is related to Plasmodium and is transmitted by the bite of the Ixodes
    dammini tick. Occasional cases have been transmitted by blood transfusions. Most patients are asymptomatic;
    symptomatic cases in reasonably healthy individuals have the features listed in the question stem. Rare severe
    cases, which may be fatal, may develop in severely debilitated or asplenic individuals and can be accompanied
    by severe hemolysis (up to 30% of RBCs may have the parasites) with subsequent hemoglobinuria, hemolysis,
    and renal failure.
    Leishmania donovania(choice B) is an intracellular tissue protozoan, not a blood protozoan.
    Plasmodium falciparum(choice C) and Plasmodium vivax(choice D) are malarial parasites not encountered on the Eastern seaboard. Trypanosoma cruzi(choice E) is an intracellular tissue protozoan, and does not infect blood cells.

    Rate this question:

  • 10. 

    A patient is referred to a neurologist because of ataxia. The neurologist diagnoses degeneration of the dorsal columns and dorsal roots of the spinal cord, which has caused impaired proprioception and locomotor ataxia. Which of the following organisms most likely caused this pattern of damage?

    • A.

      Haemophilus influenzae

    • B.

      Herpes simplex I

    • C.

      Neisseria gonorrhoeae

    • D.

      Neisseria meningitidis

    • E.

      Treponema pallidum

    Correct Answer
    E. Treponema pallidum
    Explanation
    The correct answer is E. The findings described are those of tabes dorsalis, a form of tertiary syphilis caused
    by Treponema pallidum. Tabes dorsalis and other forms of tertiary syphilis are now uncommon in this country,
    possibly because the common use of antibiotics may "treat" many unsuspected cases of syphilis.
    Haemophilus influenzae(choice A) and Neisseria meningitidis(choice D) can cause meningitis.
    Herpes simplex I (choice B) can cause an encephalitis that typically involves the frontal and temporal lobes.
    Neisseria gonorrhoeae(choice C) causes gonorrhea, which usually does not involve the CNS.

    Rate this question:

  • 11. 

    Which of the following organisms would be most likely to cause an outbreak of enteritis in a day care center in the United States?

    • A.

      Helicobacter jejuni

    • B.

      Salmonella typhi

    • C.

      Shigella species

    • D.

      Vibrio cholerae

    • E.

      Yersinia enterocolitica

    Correct Answer
    E. Yersinia enterocolitica
    Explanation
    The correct answer is E.Yersinia enterocolitica is an important cause of “mini-epidemics” of
    pediatric diarrhea. Adults can also be affected, but less commonly than children. Some diarrheal cases are
    severe (and occasionally fatal) and may be complicated by severe dysentery, appendicitis, or chronic relapsing
    ileocolitis that may require antibiotics to shorten the course.
    Helicobacter jejuni(choice A) is an important cause of hospital-acquired diarrhea.
    Salmonella typhi(choice B) causes typhoid fever. Shigella species (choice C) causes epidemics of dysentery in military camps. Vibrio cholerae(choice D) causes cholera.

    Rate this question:

  • 12. 

    A 60-year-old woman has been hospitalized for three weeks with widely metastatic ovarian adenocarcinoma, and she becomes septic with vancomycin-resistant enterococcus. What is the mechanism of vancomycin resistance in this organism?

    • A.

      Acetylation of antibiotic

    • B.

      Altered drug-binding protein

    • C.

      β-lactamase production

    • D.

      Decreased ribosomal binding of antibiotic

    • E.

      Formation of novel cell wall peptide bridges

    Correct Answer
    E. Formation of novel cell wall peptide bridges
    Explanation
    The correct answer is E. The antibiotic property of vancomycin depends upon its ability to bind D-ala-D-ala,
    which is vital in the synthesis of peptidoglycan peptide bridges. Vancomycin-resistant enterococci instead utilize
    D-lactic acid in their peptide bonds, and thus are resistant to the action of vancomycin.
    Enzymatic deactivation of antibiotics is a common mode of resistance to aminoglycosides, chloramphenicol, and
    sulfonamides. The most common chemical modifications are acetylation (choice A), adenylation, and
    phosphorylation.
    Penicillin that has penetrated the cell membrane is kept in place by binding to penicillin binding proteins (PBPs).
    Penicillin resistance may be conferred through altered PBP affinity or number (choice B). Vancomycin
    resistance does not depend on PBPs.
    β-lactamase (choice C) is a bacterial enzyme that inactivates some of the penicillins and cephalosporins,
    but has no activity on vancomycin.
    Modified ribosomes with decreased antibiotic affinity (choice D) confer resistance to protein synthesis-inhibiting
    antibiotics, including tetracycline, minocycline, doxycycline and the macrolide antibiotics. Vancomycin is not a
    protein synthesis-inhibiting antibiotic.

    Rate this question:

  • 13. 

    A 2-year-old boy has surgery to correct a urinary tract obstruction. Post-operatively, with an indwelling urinary catheter in place, he develops a urinary tract infection. Urine culture grows out a lactose-negative, oxidase-positive, gram-negative rod. Which of the following agents is the most likely cause of this boy's urinary tract infection?

    • A.

      Candida albicans

    • B.

      Enterococcus faecalis

    • C.

      Escherichia coli

    • D.

      Klebsiella pneumoniae

    • E.

      Proteus mirabilis

    • F.

      Pseudomonas aeruginosa

    • G.

      Staphylococcus saprophyticus

    Correct Answer
    F. Pseudomonas aeruginosa
    Explanation
    The correct answer is F.Pseudomonas aeruginosa is a gram-negative rod. It can easily be distinguished from
    the family Enterobacteriaceae because Pseudomonas is oxidase positive. It is an opportunistic pathogen that
    has an increased chance of causing urinary tract infections in patients with indwelling catheters, or who are on
    antibiotics.
    Candida albicans(choice A) is a yeast that can cause urinary tract infections in poorly controlled diabetics,
    because glucose in the urine enhances its growth.
    Enterococcus faecalis(choice B) is a gram-positive coccus that commonly causes urinary tract infections in
    elderly men with prostate problems.
    Escherichia coli(choice C) is a lactose-positive, oxidase-negative, gram-negative rod that is the most common
    cause of community acquired urinary tract infections.
    Klebsiella pneumoniae(choice D) is a lactose-positive, oxidase-negative, gram-negative rod that can cause
    urinary tract infections in poorly controlled diabetics because glucose in the urine enhances its growth.
    Proteus mirabilis(choice E) is a gram-negative rod that is a member of family Enterobacteriaceae. It is
    lactose-negative, but contains urease, which splits urea to yield ammonia, which in turn, raises the pH of the
    urine, creating a more hospitable environment for the organism. Patients with stones are at increased risk for
    this organism because it is able to hide in the stones (conversely, Proteus predisposes for the development of
    stones).
    Staphylococcus saprophyticus(choice G) is a catalase-positive, coagulase-negative, gram-positive coccus that
    causes urinary tract infections in young women.

    Rate this question:

  • 14. 

    A 65-year-old Laotian immigrant has developed thick, erythematous nodules on her ears and nose with significant associated sensory loss. The nodules have grown slowly over the course of many years. Biopsy of the lesions shows dermal granulomas with giant cells but no acid-fast bacteria. Culture on blood agar and Lowenstein-Jensen medium shows no growth. These findings are most consistent with which of the following diagnoses?

    • A.

      Cutaneous leishmaniasis

    • B.

      Onchocerciasis

    • C.

      Rhinoscleroma

    • D.

      Smallpox

    • E.

      Tuberculoid leprosy

    Correct Answer
    E. Tuberculoid leprosy
    Explanation
    The correct answer is E.Mycobacterium leprae is endemic to parts of Africa, Asia, and South America.
    Tuberculoid leprosy is an indolent disease, typically affecting cooler parts of the body, such as the nose and
    ears, producing dermal granulomas with very rare, acid-fast bacilli and damage to peripheral nerves. This is in
    marked contrast to lepromatous leprosy, which is progressive and invasive and generally characterized by the
    presence of numerous acid-fast bacteria in a histiocytic, but non-granulomatous tissue response. M. leprae has
    not been grown in any culture medium.
    Cutaneous leishmaniasis (choice A) is due to Leishmania species that show a worldwide distribution. The
    infection is transmitted through a sandfly bite, and the skin lesion is typically ulcerated. Histology shows
    intracellular parasites within the dermis and epidermis. Granulomas are not formed.
    Onchocerciasis (choice B), or river blindness, is a roundworm infection transmitted by black flies of Africa and
    South America. The microfilaria grow at the site of inoculation, and cause an inflamed subcutaneous nodule.
    The organism is seen on tissue sections.
    Rhinoscleroma (choice C) is a destructive granulomatous infection of the nasopharynx caused by Klebsiella
    rhinoscleromatis. Gram-negative rods can be cultured from the lesions.
    Smallpox (choice D), the infection caused by the variola virus (a DNA poxvirus), produces malaise, headaches,
    and a macular/pustular rash involving the face and distal extremities. Smallpox has been eradicated through
    worldwide vaccination.

    Rate this question:

  • 15. 

    Which of the following organisms would most likely be isolated from the vagina of a normal 5 year-old girl?

    • A.

      Candida

    • B.

      Lactobacillus

    • C.

      Neisseria

    • D.

      Staphylococcus

    • E.

      Streptococcus

    Correct Answer
    D. Staphylococcus
    Explanation
    The correct answer is D. The vagina of prepubertal girls and post-menopausal women is colonized by colonic
    and skin bacteria, including Staphylococcus epidermidis, which is normally found on the skin.
    The vagina of women of child-bearing age tends to be colonized by Lactobacillus (choice B) species, yeasts
    such as Candida (choice A), and Streptococcus species (choice E).
    The presence of Neisseria (choice C), such as N. gonorrhoeae (the cause of gonorrhea), in the vagina of a 5
    year-old strongly suggests sexual abuse.

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  • 16. 

    A 58-year-old homeless man is brought in to the emergency room by police. His temperature is 101.8°F rectally, and he smells of cheap wine. Physical examination is remarkable for dullness to percussion, increased vocal fremitus, and decreased breath sounds and wet crackles on the right. Chest x-ray reveals consolidation of the right upper lobe. One of the nurses notes the production of thick, bloody sputum when the man coughs. The most likely causative organism is

    • A.

      Haemophilus influenzae

    • B.

      Klebsiella pneumoniae

    • C.

      Pseudomonas aeruginosa

    • D.

      Staphylococcus aureus

    • E.

      Streptococcus pneumoniae

    Correct Answer
    B. Klebsiella pneumoniae
    Explanation
    The correct answer is B. Although it is far better to rely on microbiology lab results than physical exam and
    history, some classic clues for an infection with Klebsiella pneumoniae are a patient who is an elderly alcoholic
    or diabetic, "currant-jelly" sputum (containing blood clots), and lobar pneumonia.
    Hemophilus influenzae(choice A) is a frequent cause of community-acquired pneumonia, but does not
    classically produce lobar pneumonia or currant-jelly sputum.
    Pseudomonas aeruginosa(choice C) classically produces greenish sputum, and is associated with cystic fibrosis
    rather than alcoholism.
    Staphylococcus aureus(choice D) often produces pulmonary abscess, but may also produce an ordinary
    bronchopneumonia.
    Streptococcus pneumoniae(choice E) is a classic cause of lobar pneumonia, but does not produce currant-jelly
    sputum or show a particular predilection for alcoholics.

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  • 17. 

    An archaeologist who has been excavating a very old Middle Eastern site develops a pustule on his hand. The pustule then ruptures to form a black eschar surrounded by expanding brawny edema. Which of the following organisms is the most likely cause of this condition?

    • A.

      Bacillus anthracis

    • B.

      Borrelia burgdorferi

    • C.

      Francisella tularensis

    • D.

      Spirillium minus

    • E.

      Yersinia pestis

    Correct Answer
    A. Bacillus anthracis
    Explanation
    The correct answer is A. Anthrax forms extremely stable spores and has, in fact, been encountered in very old
    (i.e., deep) dirt in Israeli and Arabian excavation sites. The causative organism, Bacillus anthracis, is found in
    many animal species, and humans can acquire the organism either through contact with the animals or from
    locally contaminated soil. The pustule described in the question stem is called a "malignant pustule" and may
    be accompanied by lymphadenopathy. Most cases remain localized, but death can occur as the result of
    complications such as bacteremia, meningitis, and pneumonia. Borrelia burgdorferi(choice B) causes Lyme disease. In the first stage of this disease, there is a localized
    expanding erythematous rash rather than an eschar. Furthermore, Lyme disease occurs in the U.S., Europe,
    and Asia, corresponding to the distribution of Ixodid ticks, which spread the infection.
    Francisella tularensis(choice C) causes tularemia. The organism can persist for weeks to months but does not
    form spores that could survive for hundreds or thousands of years. Also, most human cases occur in the
    endemic areas of the U.S.
    Spirillium minus(choice D) is one of the causes of rat-bite fever. There is no indication the man was bitten by a
    rat.
    Yersinia pestis(choice E) causes bubonic plague. This organism does not form stable spores that could survive
    for extended periods of time.

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  • 18. 

    A 33-year-old woman who has recently returned from several years in the Peace Corps in Guatemala presents with severe, acute, right upper quadrant abdominal pain. She recalls that before coming back to the United States, she had several months of bloody diarrhea. CT scan of the liver demonstrates lesions that are interpreted to be abscesses. Which of the following organisms is the most likely cause of her illness?

    • A.

      Ascaris lumbricoides

    • B.

      Entamoeba histolytica

    • C.

      Enterobius vermicularis

    • D.

      Salmonella typhi

    • E.

      Shigella species

    Correct Answer
    B. Entamoeba histolytica
    Explanation
    The correct answer is B. The patient probably has hepatic amebiasis, which is a life-threatening complication of
    intestinal infection with Entamoeba histolytica.E. histolytica is transmitted via the fecal-oral route. Intestinal
    colonization (which may be asymptomatic) always precedes infection of the liver. The abscesses generally
    contain necrotic debris, with amoebae located along the edges of the abscess.
    Ascaris lumbricoides(choice A) can cause intestinal obstruction, but does not usually cause hepatic abscesses.
    Enterobius vermicularis(choice C) is the pinworm, which inhabits the rectum.
    Bloody diarrhea can also be seen with some strains of Salmonella (choice D) and Shigella (choice E), but these organisms do not usually cause hepatic abscesses. Bacterial causes of hepatic abscesses include E. coli, Klebsiella, Streptococcus, Staphylococcus, Bacteroides, and Pseudomonas.

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  • 19. 

    A previously healthy 11-year-old girl develops a gastrointestinal infection with cramping and watery stools. After several days, she begins to pass blood per rectum, and is hospitalized for dehydration. In the hospital, she is noted to have decreasing urine output with rising blood urea nitrogen (BUN). Total blood count reveals anemia and thrombocytopenia, and the peripheral smear is remarkable for fragmented red cells (schistocytes). Infection with which of the following bacterial genera is most likely responsible for this syndrome?

    • A.

      Campylobacter

    • B.

      Clostridium

    • C.

      Salmonella

    • D.

      Shigella

    • E.

      Vibrio

    Correct Answer
    D. Shigella
    Explanation
    The correct answer is D. This patient has developed hemolytic-uremic syndrome (HUS), a complication of the
    Shiga toxin or Shiga-like toxin: exotoxins released by Shigella species and the enterohemorrhagic E.coli. HUS in
    children usually develops after a gastrointestinal or flu-like illness, and is characterized by bleeding, oliguria,
    hematuria and microangiopathic hemolytic anemia. Presumably the Shiga toxin is toxic to the microvasculature,
    producing microthrombi that consume platelets and RBCs, and may fragment the red cell membrane.
    The incorrect choices are all bacteria which may produce an enterocolitis, but do not elicit HUS.
    A long-term consequence of Campylobacter (choice A) infection is a reactive arthritis or full-blown Reiter's
    syndrome.
    Clostridial enterocolitis is produced by Clostridium difficile(choice B), a normal inhabitant of the gut that
    produces pseudomembranous colitis when other gut flora are suppressed by treatment with antibiotics.
    In the United States, Salmonella infections (choice C) are almost all non-typhoid inflammatory diarrhea,
    producing a simple enterocolitis that may proceed to sepsis in some cases. Typhoid fever (produced by
    Salmonella typhi and S. paratyphi) produces a protracted illness that progresses over several weeks and
    includes rash and very high fevers, but not HUS.
    Vibrio (choice E) infections produce copious amounts of watery diarrhea, and the major risk of cholera and
    other Vibrio enteritides is shock due to hypovolemia or electrolyte loss.

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  • 20. 

    A 20-year-old man visiting Europe dives into a secluded natural pool. He later develops meningoencephalitis localized to the base of his brain. The causative organism is found to be the free-living amoeba, Naegleria fowleri. Which of the following structures did he probably damage during the dive?

    • A.

      Cribriform plate

    • B.

      Lacrimal bone

    • C.

      Mandible

    • D.

      Nasal bone

    • E.

      Zygomatic bone

    Correct Answer
    A. Cribriform plate
    Explanation
    The correct answer is A. The rare, but life-threatening, meningoencephalitis caused by Naegleria fowleri
    apparently ensues when the organism enters the body through the nasal mucosa and crosses the cribriform
    plate to enter the brain via the olfactory nerves. It has been speculated that trauma to the nose during diving
    predisposes to this infection.
    The lacrimal bone (choice B) of the medial aspect of the orbit does not appear to be involved in amoebic
    infection. The mouth and its associated bones, including the mandible (choice C), do not appear to be a route of infection
    for this organism.
    The nasal bones (choice D), located on the superior surface of the nose, do not appear to be a route of
    infection for this organism.
    The zygomatic bone (choice E), which forms the cheekbone, does not appear to be a route of infection for this
    organism.

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  • 21. 

    A 1-year-old girl presents with a 2-day history of fever, vomiting, and watery, nonbloody diarrhea. On physical exam, she appears dehydrated. Which of the following best describes the most likely infecting organism?

    • A.

      It has a complex double-stranded DNA genome

    • B.

      It has a partially double-stranded circular DNA genome

    • C.

      It has a segmented, double-stranded RNA genome

    • D.

      It has a single-stranded circular RNA genome

    • E.

      It has a single-stranded RNA genome

    Correct Answer
    C. It has a segmented, double-stranded RNA genome
    Explanation
    The correct answer is C. Rotavirus is the most common cause of gastroenteritis in children between 3 months
    and 2 years of age. It is most prevalent in the winter. Rotavirus, one of the reoviruses, looks like a wheel (which
    ROTAtes) and possesses a double-shelled icosahedral capsid with no envelope. Its genome consists of 11
    segments of double-stranded RNA.
    A complex double-stranded DNA genome (choice A) is found in enteric adenoviruses, the third most common
    cause of gastroenteritis in infants and children. This organism possesses an icosahedral nucleocapsid.
    A partially double-stranded circular DNA genome (choice B) is characteristic of hepatitis B. Its envelope contains
    surface antigen (HBsAg). Its capsid is icosahedral and contains the genome along with DNA-dependent DNA
    polymerase, which also has reverse transcriptase activity.
    A single-stranded circular RNA genome (choice D) is characteristic of hepatitis D virus. Its envelope consists of
    HBsAg. The virus is defective and is able to replicate only in cells infected with hepatitis B.
    A single-stranded RNA genome (choice E) is characteristic of several viruses that cause gastroenteritis in
    children, including astrovirus and Norwalk virus. Astrovirus is the second most common cause of viral
    gastroenteritis in young children. Three structural proteins form its capsid. Norwalk virus is the most common
    cause of gastroenteritis outbreaks in older children and adults. Its capsid consists of one structural protein.

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  • 22. 

    A 16-year-old male with sickle cell disease is hospitalized for a severe infection. His spleen has autosplenectomized and he has suffered from other minor infections in the past. His symptoms include fever, chills, cough, and chest pain. Bacteria from the patient's sputum yield optochin-sensitive organisms with a positive Quellung reaction. The organism is

    • A.

      Escherichia coli

    • B.

      Haemophilus influenzae

    • C.

      Klebsiella pneumoniae

    • D.

      Neisseria gonorrhoeae

    • E.

      Streptococcus pneumoniae

    Correct Answer
    E. Streptococcus pneumoniae
    Explanation
    The correct answer is E. The combination of optochin sensitivity and positive Quellung reaction are properties
    of a single organism, Streptococcus pneumoniae. The other encapsulated organisms that have
    Quellung-positive reactions are Haemophilus influenzae(choice B), Neisseria meningitidis, and Klebsiella
    pneumoniae(choice C). However, none of these organisms are optochin sensitive.
    The other choices, Escherichia coli(choice A) and Neisseria gonorrhoeae(choice D), are not encapsulated.

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  • 23. 

    Which of the following is specifically associated with infection with Schistosoma haematobium?

    • A.

      Adenocarcinoma of the bladder

    • B.

      Adenocarcinoma of the renal pelvis

    • C.

      Squamous cell carcinoma of the bladder

    • D.

      Transitional cell carcinoma of the bladder

    • E.

      Transitional cell carcinoma of the renal pelvis

    Correct Answer
    C. Squamous cell carcinoma of the bladder
    Explanation
    The correct answer is C. Carcinomas of the bladder and renal pelvis are usually transitional cell (choices D and
    E) carcinomas. However, Schistosoma haematobium infection (where Schistosomes lay eggs in the veins near
    the bladder, thereby inducing a marked inflammatory response) is associated with squamous metaplasia and squamous cell carci noma of the bladder. Some authors have suggested that medications used to kill the worms may contribute to the etiology. Adenocarcinomas of the renal pelvis and bladder (choices A and B) are rare.

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  • 24. 

    An active intravenous drug abuser presents to the emergency department with fever of 5 days' duration, a cough occasionally productive of blood, and pleuritic chest pain. Petechiae are present in his mouth and conjunctivae, and splinter hemorrhages are visible under the fingernails. Which of the following test results would most likely confirm the identity of the causative agent?

    • A.

      Antibodies to p24 capsid antigen

    • B.

      Antibodies to Trichinella spiralis antigen

    • C.

      Blood culture of a catalase-positive, novobiocin-sensitive, gram-positive coccus

    • D.

      Blood culture of a coagulase-positive, catalase positive, gram-positive coccus

    • E.

      Blood culture of a gamma-hemolytic, gram-positive coccus on bile-esculin agar

    • F.

      Blood culture of an alpha-hemolytic, optochin-resistant, gram-positive coccus

    • G.

      Blood culture of an alpha-hemolytic, optochin-sensitive, gram-positive coccus

    Correct Answer
    D. Blood culture of a coagulase-positive, catalase positive, gram-positive coccus
    Explanation
    The correct answer is D. The patient is exhibiting the classic signs of acute bacterial endocarditis. In intravenous
    drug abusers, Staphylococcus aureus is the most common causative agent, because it is the most common
    normal flora organism on the skin of these patients, and choice D is the classic description of this species.
    Catalase positivity distinguishes the genus Staphylococcus from Streptococcus, and Staphylococcus aureus is
    the only coagulase positive member of its genus.
    Antibodies to p24 capsid antigen (choice A) would be likely in an HIV-positive drug abuser, rather than a patient
    with endocarditis.
    Trichinella spiralis(choice B) can cause splinter hemorrhages during the larval migration period, but would not
    be the most likely agent given the other symptoms.
    Staphylococcus epidermidis is a catalase-positive, novobiocin-sensitive, gram-positive coccus (choice C). It is
    sometimes implicated in subacute bacterial endocarditis, but is less common as a causative agent in drug
    abusers than in normal individuals.
    Enterococci are gamma-hemolytic, gram-positive cocci that grow on bile-esculin agar (choice E). Enterococci
    can cause subacute bacterial endocarditis, chiefly after urologic instrumentation in men, rather than in
    association with intravenous drug abuse.
    Streptococcus viridans is an alpha-hemolytic, optochin-resistant, gram-positive coccus (choice F). It is implicated
    in subacute bacterial endocarditis after oral or dental treatments but would not be the most likely agent, given
    the acute nature of this patient's presentation.
    Streptococcus pneumoniae is an alpha-hemolytic, optochin-sensitive, gram-positive coccus (choice G) that
    produces cough and chest pain, but would be an unlikely cause of bacterial endocarditis.

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  • 25. 

    A 22-year-old woman presents with a 1-week history of mild lower abdominal pain and a yellowish vaginal discharge. She describes the pain as dull in nature, relieved slightly by acetaminophen and worsened by intercourse. Pelvic examination reveals a red, swollen cervix without motion tenderness. The mucosa is friable. Potassium hydroxide (KOH) mount is negative, and wet mount does not reveal clue cells. Gram's stain of the exudate reveals gram-negative cocci. Which of the following procedures would most likely lead to the correct diagnosis?

    • A.

      Culture the blood on Thayer-Martin agar

    • B.

      Order DNA probe assays of endocervical exudates

    • C.

      Order serologic tests to identify specific capsular antigens

    • D.

      Order the germ tube test

    • E.

      Order the rapid plasma reagin (RPR) test

    Correct Answer
    B. Order DNA probe assays of endocervical exudates
    Explanation
    The correct answer is B. The symptoms suggest infection with Neisseria gonorrhoeae; however, with Gram's
    stain results alone, it is not possible to distinguish the gonococcus from normal flora organisms such as
    Acinetobacter, unless gram-negative diplococci are found within polymorphonuclear leukocytes. The fastest
    and most reliable assay specific for diagnosis of gonorrhea is the use of commercial DNA probes, with results
    available in 2-4 hours. Culture on Thayer-Martin medium (choice A) is indeed the choice for culture of N. gonorrhoeae, but would yield
    slower results, and therefore be a secondary choice.
    Serology (choice C) is not a good choice because N. gonorrhoeae does not have a significant capsule (the
    meningococcus does), and furthermore, serological tests for the gonococcus have proven insensitive and
    nonspecific.
    The germ tube test (choice D) is one of the diagnostic tests for Candida albicans, but this case presentation is
    not characteristic of candidiasis.
    The RPR (choice E) is a test for reaginic (heterophilic) antibodies formed early in infection with Treponema
    pallidum, but this case presentation is not suggestive of syphilis.

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