USMLE Microbiology - Part 2

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USMLE Microbiology - Part 2 - Quiz

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Questions and Answers
  • 1. 

    A 4-year-old boy presents to the pediatric emergency department with the classic meningitis triad of fever, headache, and nuchal rigidity. A lumbar puncture is performed and analysis of the fluid shows an increase in polymorphonuclear cells, an increased protein level, and a decreased glucose level. Which of the following is the most common cause of meningitis in a child of this age with this clinical picture?

    • A. 

      Enteroviruses

    • B. 

      Haemophilus infl uenzae type B

    • C. 

      Herpes simplex virus

    • D. 

      Listeria species

    • E. 

      Streptococcus pneumoniae

    Correct Answer
    E. Streptococcus pneumoniae
    Explanation
    The correct answer is E. The results of the fluid analysis are consistent with a bacterial meningitis
    (increased polymorphonuclear cells, high protein, low glucose). The most common cause of meningitis in children aged 6 months to 6 years is Streptococcus pneumoniae.

    Answer A is incorrect. Results of the fluid analysis are not consistent with a viral meningitis. One would expect to see increased lymphocyte counts, normal protein levels, and normal sugar levels in this type of infection.

    Answer B is incorrect. Haemophilus influenzae meningitis is a less common cause of meningitis in children of this age group. Since the introduction of the Haemophilus flu vaccine, the incidence of this cause of meningitis has greatly decreased.

    C is incorrect. The clinical picture does not fit with a viral meningitis. In a viral meningitis, one would expect the fluid analysis to come back with increased lymphocytes, normal protein, and normal sugar.

    Answer D is incorrect. Listeria species is not a common cause of meningitis in this age group. It is much more commonly seen in newborns age 0–6 months and in the elderly.

    Rate this question:

  • 2. 

    A 32-year-old man presents to his doctor with painful urination and a purulent urethral discharge. The image shows cells that have been cultured from this discharge. Which of the following is the treatment of choice for this infection?

    • A. 

      Azithromycin

    • B. 

      Ceftriaxone

    • C. 

      Fluconazole

    • D. 

      Penicillin

    • E. 

      Vancomycin

    Correct Answer
    A. Azithromycin
    Explanation
    The correct answer is A. These symptoms are typical of urethritis. The most common causes of urethritis in males are Chlamydia trachomatis and Neisseria gonorrhoeae. The image shows intracellular inclusions that indicate that this man is infected with C. trachomatis; in the image, EB indicates the elementary body particles within cell walls and RB indicates the reticular body. While they may be difficult to differentiate, C. trachomatis infection induces a predominantly immunologic reaction with only some polymorphonuclear leukocytes (PMNs), while N. gonorrhoeae induces predominantly nonimmunologic inflammation with a PMN-rich infi ltrate. The treatment of choice for Chlamydia urethritis is azithromycin (macrolide family antibiotic) or doxycycline (tetracycline family antibiotic).

    Answer B is incorrect. Ceftriaxone is an effective treatment for gonorrhea, but the cephalosporin class of antibiotics is relatively ineffective against Chlamydia trachomatis.

    Answer C is incorrect. Fluconazole inhibits fungal steroid synthesis. It is used in the treatment of
    fungal infections, such as Candida albicans.

    Answer D is incorrect. Penicillin has been shown to suppress chlamydial multiplication. However, it does not eradicate the organism and thus is not the best treatment for this type of infection. Penicillin is the treatment of choice for syphilis.

    Answer E is incorrect. Vancomycin has not been shown to be effective in the treatment of chlamydial infection. It is used to treat drugresistant Staphylococcus aureus and Clostridiumdifficile.

    Rate this question:

  • 3. 

    A 43-year-old man presents with fl u-like symptoms, fevers, chills, and a productive cough. Physical examination is remarkable for pleuritic chest pain. On questioning, the patient says that he has just spent the last week on vacation in Central America. Cultures taken from the patient show a broad-based budding fungus. It is concluded that the man has systemic blastomycosis infection. Which of the following agents is the most appropriate treatment for this patient?

    • A. 

      Amphotericin B

    • B. 

      Fluconazole

    • C. 

      Itraconazole or potassium iodide

    • D. 

      Sodium stibogluconate only

    • E. 

      Topical miconazole or selenium sulfide

    Correct Answer
    A. Amphotericin B
    Explanation
    The correct answer is A. Blastomycosis can present with fl u-like symptoms, fevers, chills, productive cough, myalgia, arthralgia, and pleuritic chest pain. Some patients will fail to recover from an acute infection and progress to develop chronic pulmonary infection or widespread disseminated infection. Fluconazole or ketoconazole is used for the treatment of local blastomycosis infections, and amphotericin B is used for the treatment of systemic infections.

    Answer B is incorrect. Fluconazole or ketoconazole are effective treatments for local blastomycosis infections but are ineffective if the infection is systemic. Systemic infections require amphotericin B.

    Answer C is incorrect. Itraconazole or potassium iodide is used for the treatment of Sporothrix
    schenckii. S. schenckii is the cause of sporotrichosis. When S. schenckii is introduced into the skin, usually by a thorn prick, it causes a local pustule or ulcer with nodules along draining lymphatics (ascending lymphangitis). S. schenckii is a dimorphic fungus that has cigar-shaped budding yeast visible in pus.

    Answer D is incorrect. Sodium stibogluconate is used to treat Leishmania donovani infection.
    L. donovani presents with hepatomegaly and splenomegaly, malaise, anemia, and weight loss.
    L. donovani is transmitted via the sandfly. Microscopically, macrophages containing amastigotes are observed.

    Answer E is incorrect. Topical miconazole or selenium sulfide (Selsun) is used to treat Malassezia furfur. M. furfur is the cause of tinea versicolor. Symptoms of this infection include hypopigmented skin lesions that occur in hot and humid conditions.

    Rate this question:

  • 4. 

    73-year-old woman steps on a rusty nail while gardening in her back yard. A neighbor drives her to the emergency department, where her wound is cleaned and bandaged. A complete history and review of her medical chart indicates that her vaccinations are not up to date, and she requires one vaccination and a shot of the appropriate immunoglobulin as prophylaxis against infection. What is the mechanism of toxicity of the organism for which she was vaccinated?

    • A. 

      ADP-ribosylation of a G protein, increasing chloride secretion

    • B. 

      ADP-ribosylation of an elongation factor, causing disrupted protein synthesis

    • C. 

      Binding to the MHC II receptor and T-lymphocyte receptor, causing cytokine synthesis

    • D. 

      Blocking release of the inhibitory neurotransmitter glycine

    • E. 

      Blocking the release of acetylcholine

    • F. 

      Lysis of RBCs

    • G. 

      Stimulation of guanylate cyclase

    Correct Answer
    D. Blocking release of the inhibitory neurotransmitter glycine
    Explanation
    The correct answer is D. The penetrating wound from the rusty nail puts this patient at risk for infection with Clostridium tetani, whose symptoms are caused by the tetanus toxin. This toxin blocks the release of glycine from Renshaw cells in the spinal cord and results in “lockjaw” and other similar symptoms. Tetanus re-vaccination is required approximately every 10 years to ensure adequate blood levels of protective antibodies.

    Answer A is incorrect. The Vibrio cholera toxin ADP-ribosylates a G protein in the intestine, increasing adenylate cyclase activity and causing pumping of water and chloride ions into the gut lumen. Its most characteristic symptom is voluminous “rice water” diarrhea. The heat-labile toxin of Escherichia coli has the same mechanism of action.

    Answer B is incorrect. The Corynebacterium diphtheriae toxin inactivates the elongation factor EF-2 by ADP ribosylation, disrupting protein synthesis. It causes pharyngitis and “pseudomembrane” in the throat. Although the patient’s prior vaccinations most likely included vaccination for diphtheria as well, this infection is less likely than tetanus to occur with a penetrating wound.

    Answer C is incorrect. Superantigens such as the Staphylococcus aureus toxin and the Streptococcus
    pyogenes erythrogenic toxin bind to the MHC class II receptor and the T-lymphocyte receptor, resulting in cytokine release and sometimes toxic shock syndrome.

    Answer E is incorrect. The Clostridium botulinum toxin inhibits the release of acetylcholine, resulting in anticholinergic symptoms and even CNS. C. botulinum is most often found in canned food and honey (resulting in “floppy baby” syndrome when consumed by young children).

    Answer F is incorrect. The streptolysin O toxin of Streptococcus pyogenes is a hemolysin.

    Answer G is incorrect. The heat-stable toxin of Escherichia coli stimulates guanylate cyclase.

    Rate this question:

  • 5. 

    A 50-year-old man develops nonbloody watery diarrhea while working as an aid worker in a refugee camp in Bangladesh. He arrived in the area 2 days ago. A stool smear shows no WBCs. He subsequently develops dehydration and electrolyte abnormalities leading to cardiac and renal failure. Which of the following organisms is the most likely cause of this man’s enterocolitis?

    • A. 

      Clostridium diffi cile

    • B. 

      Giardia lamblia

    • C. 

      Helicobacter pylori

    • D. 

      Salmonella species

    • E. 

      Vibrio cholerae

    Correct Answer
    E. Vibrio cholerae
    Explanation
    The correct answer is E. Vibrio cholerae causes watery stools, often called rice-water stool. This illness is not accompanied by abdominal pain, but the symptoms are due to dehydration leading to electrolyte imbalances. Cholera toxin causes uncontrolled stimulation of adenylate cyclase; the resulting excess of cAMP causes uncontrolled secretion of chloride and water (due to the osmotic gradient), resulting in extremely watery diarrhea accompanied by electrolyte imbalances.

    Answer A is incorrect. Clostridium difficile causes severe nonbloody diarrhea associated with pseudomembranes. Diarrhea may be bloody when very severe mucosal ulceration occurs. C. difficile infection is associated with previous antibiotic treatment.

    Answer B is incorrect. Giardia lamblia is a protozoan parasite that is a frequent cause of nonbloody diarrhea. The stools are usually foul-smelling and contain fat (steatorrhea). It is less severe than the diarrhea caused by Vibrio cholerae.

    Answer C is incorrect. Helicobacter pylori infection causes gastritis and would not cause the
    symptoms described in this patient.

    Answer D is incorrect. Salmonella species invade the mucosa to cause bloody diarrhea and can be acquired from poultry, meat, and eggs.

    Rate this question:

  • 6. 

    Twenty-four hours after placement of a catheter, a hospitalized patient develops a fever and chills. Within 1 hour her systolic blood pressure falls 30 points and she develops swelling in her extremities. Despite valiant efforts by the hospital staff, the patient dies. X-ray of the patient’s lungs taken only hours before she passed away shows pulmonary edema. Which of the following mediators of this patient’s disease process is most likely responsible for the pathology described?

    • A. 

      C3a

    • B. 

      C5a

    • C. 

      Hageman factor

    • D. 

      γ-Interferon

    • E. 

      Interleukin-1

    • F. 

      Nitric oxide

    Correct Answer
    A. C3a
    Explanation
    The correct answer is A. This patient suffered
    from shock, most likely endotoxic shock due to
    gram-negative bacteremia. The endotoxin lipopolysaccharide
    (LPS) is found in the cell
    wall of gram-negative bacteria. LPS activates
    the alternative pathway of the complement cascade.
    The patient’s pulmonary edema contributes
    to the acute respiratory distress syndrome
    that accompanies septic shock. The C3a component
    of the complement cascade contributes
    to the hypotension and edema seen in endotoxic
    shock.
    Answer B is incorrect. The C5a component of
    the complement cascade, activated by endotoxin,
    functions in neutrophil chemotaxis.
    Answer C is incorrect. Endotoxin can directly
    activate Hageman factor, activating the coagulation
    cascade and leading to disseminated intravascular
    coagulation.
    Answer D is incorrect. γ-Interferon is produced
    by T lymphocytes and, among other
    functions, activates tumoricidal macrophages.
    Answer E is incorrect. The cytokine interleukin-
    1, released by macrophages activated by
    endotoxin, causes fever.
    Answer F is incorrect. Nitric oxide, released
    by macrophages activated by endotoxin, causes
    hypotension (shock).

    Rate this question:

  • 7. 

    A 31-year-old pregnant woman comes to the physician because of painful vesicular lesions that have recently appeared on her genitalia. A positive result on which of the following diagnostic tests would mean that her baby is at risk for congenital anomalies?

    • A. 

      Anti-hepatitis B surface antibody test

    • B. 

      Giemsa stain for cytoplasmic inclusions

    • C. 

      Monospot test

    • D. 

      Tzanck smear for multinucleated giant cells

    • E. 

      Weil-Felix test

    Correct Answer
    D. Tzanck smear for multinucleated giant cells
    Explanation
    The correct answer is D. ToRCHeS is an acronym
    for organisms that can cross the placenta
    and cause congenital anomalies: Toxoplasmosis,
    Rubella, Cytomegalovirus, HIV/
    Herpes, and Syphilis. Genital lesions suggest a
    sexually transmitted disease. The Tzanck test is
    a smear of an opened skin vesicle that detects
    multinucleated giant cells, indicative of HSV
    types 1 and 2 or VZV. HSV-1, HSV-2, and
    VZV may all be transmitted vertically to the fetus.
    Remember: “Tzanck heaven I don’t have
    herpes!” In the United States today, CMV is
    the most common cause of congenital abnormalities.
    Answer A is incorrect. The presence of anti-
    HB surface antibody indicates immunity to the
    hepatitis B virus, either by previous exposure or
    by vaccination. While active or chronic hepatitis
    B can be vertically transmitted to the fetus,
    it does not cause congenital anomalies. Furthermore,
    hepatitis B is not associated with
    genital lesions.
    Answer B is incorrect. Cytoplasmic inclusions
    seen on Giemsa or fl uorescent antibodystained
    smear suggest Chlamydia trachomatis.
    Although Chlamydia can be vertically transmitted
    to the fetus at delivery, it is not associated
    with congenital anomalies or with painful
    vesicular lesions. It can, however, cause blindness
    and pneumonia in the newborn and
    should be treated during pregnancy.
    Answer C is incorrect. The monospot test detects
    heterophile antibodies by the agglutination
    of sheep RBCs, indicative of EBV infection.
    While EBV can cause mononucleosis
    and Burkitt’s lymphoma, it is not one of the
    ToRCHeS organisms and does not transmit
    pavertically
    to the fetus. Moreover, EBV does not
    produce genital lesions.
    Answer E is incorrect. The Weil-Felix test uses
    Proteus antigen to test for antirickettsial antibodies,
    indicative of typhus or Rocky Mountain
    spotted fever. Rickettsia species can cause
    headache, fever, and rash; however, they do
    not produce genital lesions.

    Rate this question:

  • 8. 

    A 13-year-old girl who returned a few days ago from a school camping trip in North Carolina is home ill from school. She tells her parents that she has a headache and the chills. Over the next few days, she develops a rash that begins on her palms and soles, but spreads inward to her wrists and ankles and then to her trunk. Her worsening condition leads her parents to take her to the emergency department, where a blood test reveals antibodies that react with the Proteus antigen. This patient is most likely infected with which of the following?

    • A. 

      Borrelia burgdorferi

    • B. 

      Coxiella burnetti

    • C. 

      Coxsackievirus A

    • D. 

      Rickettsia rickettsii

    • E. 

      Rickettsia typhi

    • F. 

      Treponema pallidum

    Correct Answer
    D. Rickettsia rickettsii
    Explanation
    The correct answer is D. This patient most
    likely has Rocky Mountain spotted fever, as indicated
    by the rash on her palms and soles and
    the inward, “centripetal” pattern of spread.
    Other supporting evidence are the accompanying
    headache and fever, and a positive Weil-
    Felix reaction, which is a cross-reaction of certain
    anti-rickettsial antibodies with the Proteus
    antigen. Rocky Mountain spotted fever is
    caused by the rickettsial organism Rickettsia
    rickettsii, and is endemic to the east coast of
    the United States. It is transmitted by the
    Ixodes tick, thus the patient probably acquired
    it during her recent camping trip.
    Answer A is incorrect. Borrelia burgdorferi
    causes Lyme disease. The rash of Lyme disease
    is typically a bull’s-eye type rash, with negative
    Weil-Felix reaction.
    Answer B is incorrect. Coxiella burnetti is also
    a rickettsial organism. It is transmitted by aerosols,
    causes Q fever, and has no associated rash
    or positive Weil-Felix reaction.
    Answer C is incorrect. Coxsackie A is an RNA
    virus that causes hand, foot, and mouth disease,
    which can also present with a rash on the
    palms and soles, in addition to oral and occasionally
    genital lesions. However, the positive
    Weil-Felix reaction in this case and recent history
    of a camping trip point to Rickettsia rickettsiae
    as a more likely causative organism in
    this case.
    Answer E is incorrect. Rickettsia typhi causes
    endemic typhus, and is transmitted by fl eas.
    The rash of typhus is centrifugal; it spreads outward,
    not inward as in this case.
    Answer F is incorrect. Treponema pallidum is
    the spirochete that causes syphilis, a sexually
    transmitted disease. Although syphilis can also
    present with a rash on the palms and soles, this
    patient has no history of a sexual encounter
    that would put her at risk for this disease.

    Rate this question:

  • 9. 

    A 53-year-old obese man with poorly controlled non-insulin-dependent diabetes mellitus presents with fever to 39.6° C (103.2° F), jaundice, hypotension, and acute onset of right upper quadrant pain. Right upper quadrant imaging shows multiple gallstones and cholecystitis. Urgent cholecystectomy is performed, and subsequent gall bladder fl uid and blood cultures grow aerobic, non-lactose-fermenting, oxidasepositive, gram-negative rods. Blood tests show: Hematocrit: 29% WBC count: 14,700/mm3 Platelet count: 76,000/mm3 D-dimer: 8500 ng/mL Fibrinogen levels: low Microscopic inspection of peripheral blood smear shows schistocytes and multiple helmet cells. Clinically, there is no evidence of active bleeding. What is the most appropriate treatment for this patient’s coagulopathy?   International Normalized Ratio: 3.2

    • A. 

      Amoxicillin

    • B. 

      Aztreonam

    • C. 

      Fresh frozen plasma

    • D. 

      Vancomycin

    • E. 

      Vitamin K

    Correct Answer
    B. Aztreonam
    Explanation
    The correct answer is B. This patient has leukocytosis
    and Charcot’s triad (fever, jaundice,
    right upper quadrant pain), along with the ominous
    sign of hypotension, a clear clinical picture
    of cholecystitis. In addition, he has
    Pseudomonas aeruginosa sepsis and disseminated
    intravascular coagulation (DIC). Gramnegative
    rod sepsis is the clear cause of this patient’s
    DIC, and antipseudomonal coverage
    with aztreonam is most appropriate. Aztreonam
    is a β-lactamase-resistant monobactam
    that interferes with cell wall biosynthesis by
    binding to penicillin-binding protein 3. Aztreonam
    is a potent antipseudomonal agent indicated
    for pseudomonal sepsis.
    Answer A is incorrect. Amoxicillin is an aminopenicillin
    antibiotic that interferes with cell
    wall synthesis. Although amoxicillin has an extended
    spectrum compared with penicillin
    (covering Haemophilus infl uenzae, Escherichia
    coli, Listeria, Proteus, Salmonella, and Enterococci),
    it does not provide antipseudomonal
    coverage.
    Answer C is incorrect. Use of fresh frozen
    plasma (FFP) is reserved for patients with coagulopathy
    and signs of active, life-threatening
    bleeding. Although provision of FFP will temporarily
    reverse some of this patient’s laboratory
    signs of DIC (elevated International Normalized
    Ratio, decreased fi brinogen), treatment of
    the underlying cause (ie, Pseudomonas sepsis)
    is most important.
    Answer D is incorrect. Vancomycin is an antibiotic
    used for serious multidrug-resistant,
    gram-positive infections. Major uses are for
    methicillin-resistant Staphylococcus aureus and
    moderate to severe Clostridium diffi cile infections.
    Its mechanism of action is to inhibit cell
    wall mucopeptide formation by binding the
    D-ala-D-ala portion of cell wall precursors.
    Answer E is incorrect. Coagulopathy caused
    by warfarin overdose is reversed by pharmacologic
    administration of vitamin K. This patient’s coagulopathy is caused by Pseudomonas
    sepsis, so vitamin K therapy plays no role here.

    Rate this question:

  • 10. 

    A 22-year-old woman presents to the physician with vaginal itching and burning. On examination, she has a foul-smelling greenish discharge. A swab sample is taken and a wet mount slide is prepared; results are shown in the image. Which of the following medications should be prescribed for this patient?

    • A. 

      Metronidazole

    • B. 

      Nifurtimox

    • C. 

      Quinine

    • D. 

      Sodium stibogluconate

    • E. 

      Sulfadiazine and pyrimethamine

    Correct Answer
    A. Metronidazole
    Explanation
    The correct answer is A. Trichomonas vaginalis
    is the cause of vaginitis. Symptoms of vaginitis
    include a foul-smelling greenish discharge,
    itching, and burning. T. vaginalis is
    transmitted sexually. On microscopic wet
    mount, one fi nds trophozoites. Metronidazole
    is used to treat T. vaginalis infection.
    Answer B is incorrect. Nifurtimox is used to
    treat Trypanosoma cruzi. T. cruzi infection
    causes Chagas’ disease, a condition in which
    the heart is enlarged and fl accid. T. cruzi is
    transmitted via the reduviid bug. Microscopic
    examination reveals fl agellated trypomastigotes
    in the blood and nonmotile amastigotes in tissue
    culture.
    Answer C is incorrect. Quinine is used to treat
    babesiosis. Babesia species present with a malaria-
    like syndrome. Babesiosis is transmitted
    by the Ixodes tick. On microscopic examination,
    one observes no red blood cell pigment
    and the Maltese cross-appearing parasite.
    Answer D is incorrect. Sodium stibogluconate
    is used to treat Leishmania donovani infection.
    L. donovani presents with hepatomegaly and
    splenomegaly, malaise, anemia, and weight
    loss. L. donovani is transmitted via the sandfl y.
    Microscopically, macrophages containing
    amastigotes are observed.
    Answer E is incorrect. Sulfadiazine and pyrimethamine
    are used to treat toxoplasmosis.
    Toxoplasma gondii infection presents with
    brain abscesses in HIV-positive patients and
    with birth defects. T. gondii is transmitted via
    cysts in raw meat or cat feces. The defi nitive
    stage (sexual stage) occurs in cats. Microscopically,
    acid-fast staining cysts are found.

    Rate this question:

  • 11. 

    A 40-year-old man goes on a camping vacation with his family. One day after swimming in a freshwater lake near the camp site, he develops nausea and vomiting and starts to behave irrationally. His family takes him to the emergency department, where blood samples are taken and a spinal tap is performed. He is diagnosed with a rapidly progressing meningoencephalitis and dies shortly thereafter. Which of the following protozoa was most likely the cause of the man’s illness?

    • A. 

      Cryptosporidium species

    • B. 

      Entamoeba histolytica

    • C. 

      Leishmania donovani

    • D. 

      Naegleria fowleri

    • E. 

      Plasmodium falciparum

    Correct Answer
    D. Naegleria fowleri
    Explanation
    The correct answer is D. Naegleria fowleri
    presents with a rapidly progressing meningoencephalitis
    that can progress to coma or death
    within 6 days. Other symptoms include nausea,
    vomiting, and irrational behavior. Transmission
    occurs through swimming in freshwater
    lakes. Microscopic analysis will reveal
    amebas in the spinal fl uid. Unfortunately,
    there is no treatment for N. fowleri.
    Answer A is incorrect. Cryptosporidium species
    infection presents with severe diarrhea in
    HIV-positive patients and mild watery diarrhea
    in HIV-negative patients. Cryptosporidium species
    are transmitted via cysts in water (fecal-oral
    transmission). Microscopically, acid-fast staining
    cysts are found. Unfortunately, there is no
    treatment available for Cryptosporidium species
    Infection; however, in healthy patients,
    cryptosporidiosis is self-resolving.
    Answer B is incorrect. Entamoeba histolytica
    infection presents with bloody diarrhea (dysentery),
    abdominal cramps with tenesmus, and
    pus in the stool. It can also cause right upper
    quadrant pain and liver abscesses. E. histolytica
    is transmitted via cysts in water (fecal-oral transmission).
    On microscopy, one observes amebas
    with ingested RBCs. Treatment for E. histolytica
    infection includes metronidazole and iodoquinol.
    Answer C is incorrect. Leishmania donovani
    infection presents with hepatomegaly and
    spleno megaly, malaise, anemia, and weight
    loss. L. donovani is transmitted via the sandfl y.
    Microscopically, macrophages containing
    amastigotes are observed. Sodium stibogluconate
    is used to treat L. donovani infection.
    Answer E is incorrect. The Plasmodium falciparum
    parasite is responsible for causing malaria.
    It is spread by the Anopheles mosquito.
    Diagnosis of Plasmodium falciparum infection
    is made through a blood smear.

    Rate this question:

  • 12. 

    A 54-year-old man presents to the clinic with scleral icterus, hepatosplenomegaly, ascites, and a history of episodes of jaundice over the past 3 years. He was involved in an auto accident when he was 21 years old, for which he required surgery and blood transfusions. Laboratory tests show: Aspartate aminotransferase: 734 U/L Alanine aminotransferase: 846 U/L Direct bilirubin: 0.1 mg/dL Indirect bilirubin: 7.6 mg/dL Assuming a viral etiology, which of the following is the most likely cause of this patient’s illness?

    • A. 

      Hepatitis A

    • B. 

      Hepatitis C

    • C. 

      Hepatitis D

    • D. 

      Hepatitis E

    • E. 

      Hepatitis G

    Correct Answer
    B. Hepatitis C
    Explanation
    The correct answer is B. This is a classic presentation
    of chronic hepatitis C infection. It is
    a common cause of post-transfusion viral hepatitis.
    Hepatitis C is a blood-borne pathogen
    that can ultimately cause cirrhosis of the liver.
    Answer A is incorrect. Hepatitis A is transmitted
    via the fecal-oral route and does not cause
    chronic infection.
    Answer C is incorrect. Hepatitis D is transmitted
    parenterally and can cause infection only if
    its host is coinfected with hepatitis B.
    Answer D is incorrect. Hepatitis E is transmitted
    via the fecal-oral route and does not cause
    chronic infection.
    Answer E is incorrect. Hepatitis G is a transmissible
    fl avivirus that has not been shown to
    cause liver disease.

    Rate this question:

  • 13. 

    A 19-year-old college student presents to his family physician with a 1-week history of fever, headache, and painful exudative pharyngitis. Physical examination shows signifi cant lymphadenopathy of the cervical nodes and hepatosplenomegaly. Laboratory studies show a WBC count of 15,000/mm3 with 55% lymphocytes. A heterophile antibody test is positive. Which of the following is the most likely cause of this patient’s symptoms?

    • A. 

      Cytomegalovirus

    • B. 

      Epstein-Barr virus

    • C. 

      HIV infection

    • D. 

      Streptococcus pneumoniae

    • E. 

      Toxoplasma gondii

    Correct Answer
    B. Epstein-Barr virus
    Explanation
    The correct answer is B. EBV causes infectious
    mononucleosis and is a member of the
    Herpesviridae family. Mononucleosis typically
    presents with high fever, elevated WBC count,
    painful pharyngitis, and enlarged lymph nodes.
    There are two signifi cant clinical differences
    between EBV mononucleosis and that of
    CMV: exudative pharyngitis and cervical
    lymphadenopathy are commonly seen with
    EBV, but not with CMV. A positive heterophile
    antibody test is specifi c for EBV infection.
    Answer A is incorrect. CMV causes a mononucleosis
    syndrome that is very similar to the one
    caused by EBV; however, one rarely sees an exudative
    pharyngitis or cervical lymphadenopathy
    when CMV is the cause. CMV can also cause
    pneumonia and, if transmitted congenitally,
    birth defects. After infection with CMV, most
    healthy individuals are asymptomatic, but
    CMV can reactivate in the immunocompromised
    and cause a variety of illnesses in various
    organ systems.
    Answer C is incorrect. HIV is the virus that
    causes AIDS. AIDS is a syndrome characterized
    by a decline in CD4+ cell count

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  • 14. 

    A 36-year-old man comes to the physician complaining of an aching back, high fever, and vomiting of dark material. He is obviously ill and states that he has felt very poorly for approximately 1 week. Physical examination shows that the patient has a temperature of 39° C (102.2° F) and icteric sclera. The patient recently returned from a trip on safari in Africa. If a liver biopsy were done, it would show the following pathology. What are the names of the eosinophilic globules shown in this image?

    • A. 

      Councilman bodies

    • B. 

      Döhle bodies

    • C. 

      Negri bodies

    • D. 

      Pappenheimer bodies

    • E. 

      Weibel-Palade bodies

    Correct Answer
    A. Councilman bodies
    Explanation
    The correct answer is A. The disease described
    is yellow fever, caused by a member of the Flaviviridae
    family. It presents with symptoms of
    jaundice, aching pain, and high fever. Its vector
    is the mosquito. Liver biopsy can reveal
    Councilman bodies, which are eosinophilic
    globules believed to be the result of apoptosis
    of individual hepatocytes.
    Answer B is incorrect. Döhle bodies are oval
    bodies found in the neutrophils of patients
    with infections, trauma, pregnancy, or cancer.
    Answer C is incorrect. Negri bodies are
    pathognomonic for the rabies virus. They are
    eosinophilic inclusion bodies found in the cytoplasm
    of nerve cells of infected individuals.
    Answer D is incorrect. Pappenheimer bodies
    are found in RBCs in sideroblastic anemia and
    sickle cell disease. They are phagosomes containing
    ferruginous granules.
    Answer E is incorrect. Weibel-Palade bodies
    can be seen by electron microscopy in vascular
    endothelial cells. They are collections of microtubules.

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  • 15. 

    A 56-year-old man presents to the emergency department with sharp retrosternal pain radiating to his back and arms. The patient is sitting up and leaning forward. He states that the pain is less severe in this position and worsens when he lies down and takes a deep breath. He also indicates that he recently recovered from a fever and a cold. On physical examination a scratchy, leathery sound is heard at the lower left sternal border. An ECG is done and confi rms the diagnosis. Which of the following microorganisms is the most likely cause of this condition?

    • A. 

      Coronavirus

    • B. 

      Coxsackievirus

    • C. 

      Cytomegalovirus

    • D. 

      Epstein-Barr virus

    • E. 

      Staphylococcus aureus

    Correct Answer
    B. Coxsackievirus
    Explanation
    The correct answer is B. This patient presents
    with classic signs and symptoms of pericarditis.
    Pericarditis frequently follows an upper respiratory
    viral infection most commonly due to coxsackie
    B virus, which causes infl ammation of
    the pericardial membrane. Auscultation of the
    chest would reveal a pericardial friction rub
    that accounts for the scratchy, leathery sound
    heard during both systole and diastole. On
    ECG there would be diffuse ST segment elevation
    and a depression of the PR segment
    unique to pericarditis.
    Answer A is incorrect. Coronavirus is a common
    virus that causes a self-limiting cold.
    SARS-CoV, however, has been identifi ed as
    the cause of severe acute respiratory syndrome
    Answer C is incorrect. CMV causes a mononucleosis
    syndrome in young adults similar to
    that caused by EBV, which is characterized by
    fever and pharyngitis. It can also cause a severe
    infection in immunocompromised patients,
    which is characterized by retinitis, pneumonia,
    and even death.
    Answer D is incorrect. EBV causes heterophilpositive
    mononucleosis with symptoms of fever,
    fatigue, lymphadenopathy, and lymphocytosis.
    In cases of suspected EBV infection, the
    peripheral blood smear should be evaluated for
    atypical lymphocytes and a heterophile antibody
    test should be performed.
    Answer E is incorrect. Staphylococcus aureus
    is a common cause of acute bacterial endocarditis
    in intravenous drug users, and rarely
    causes pericarditis.

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  • 16. 

    A 28-year-old man comes to the physician because of worsening muscle weakness that began in his legs and feet 3 days ago, and has now spread to his arms and hands. Other than having a fl u-like illness 2 weeks ago, the patient has been in good health. Cerebrospinal fl uid analysis shows an increased protein concentration, a normal cell count, and a normal glucose level. An infection with which of the following organisms is the most likely cause of the nervous system syndrome described in this patient?

    • A. 

      Candida albicans

    • B. 

      Legionella pneumophila

    • C. 

      Mycoplasma pneumoniae

    • D. 

      Pseudomonas aeruginosa

    • E. 

      Streptococcus pneumoniae

    Correct Answer
    C. Mycoplasma pneumoniae
    Explanation
    The correct answer is C. The syndrome described
    is Guillain-Barré syndrome, a common
    cause of acute peripheral neuropathy that results
    in progressive weakness over a period of
    days. Although one-third of patients report no
    history of an antecedent infection, the other
    two-thirds have recently experienced an acute
    gastrointestinal or infl uenza-like illness prior to
    developing the neuropathy. The most common
    epidemiologic associations involve infections
    with Campylobacter jejuni, Hae mophilus infl uenzae,
    CMV, EBV, Mycoplasma pneumoniae,
    and VZV. Laboratory abnormalities associated
    with Guillain-Barré syndrome include elevated
    gamma-globulin, decreased nerve conduction
    velocity indicative of demyelination, and albuminocytologic
    dissociation (CSF shows increased
    protein concentration with normal cell
    count in the setting of normal glucose). Although
    the organisms listed frequently precede
    the syndrome, there has never been any consistent
    demonstration of any single infectious
    agent in the peripheral nerves of these patients,
    and the cause of the disease is thought to be
    mediated by hypersensitive T lymphocytes.
    Answer A is incorrect. Although immunocompromised
    patients may be at greater risk for the
    organisms that are commonly associated with
    Guillain-Barré syndrome, there is no indication
    this patient has a weak immune system.
    Furthermore, Candida albicans does not have
    any association with the syndrome.
    Answer B is incorrect. While Legionnaire’s
    disease has been at least anecdotally associated
    with Guillain-Barré syndrome, an otherwise
    healthy 28-year-old man would not be expected
    to develop an infection with Legionella
    pneumophila.
    Answer D is incorrect. Pseudomonas aeruginosa
    is not an organism associated with Guillain-
    Barré syndrome.
    Answer E is incorrect. Pneumococcal pneumonia
    is not associated with the development
    of Guillain-Barré syndrome.

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  • 17. 

    A 55-year-old man comes to his physician with a tender, swollen, and erythematous left knee. He has limited range of motion in his leg. On aspiration of the synovial fl uid from his knee, the fl uid is found to be yellow and cloudy and has 150,000 neutrophils/mm3. Gram stain of the aspirate shows gram-positive cocci in clusters. The organism most likely responsible for this patient’s symptoms has which of the following properties?

    • A. 

      Catalase-positive and coagulase-negative

    • B. 

      Catalase-positive and coagulase-positive

    • C. 

      Glycoprotein capsule

    • D. 

      α-Hemolysis

    • E. 

      Soluble in bile

    Correct Answer
    B. Catalase-positive and coagulase-positive
    Explanation
    The correct answer is B. This man is suffering
    from septic arthritis, commonly characterized
    by a swollen, tender, and erythematous joint.
    The organism most commonly responsible for
    this infection is Staphylococcus aureus. The infection
    results from the invasion of the bacteria
    into the synovial fl uid. The diagnosis of septic
    arthritis requires aspiration of the synovial
    fl uid, which appears yellow and turbid with a
    predominance of neutrophils. When Staphylococcus
    is the causative agent, Gram stain and
    culture of the synovial fl uid show gram-positive
    cocci in clusters. S. aureus is catalase-positive
    and coagulase-positive.
    Answer A is incorrect. While S. aureus is indeed
    catalase-positive, it is not coagulase-negative.
    This answer instead describes the properties
    of S. epidermidis.
    Answer C is incorrect. S. aureus does not possess
    a glycoprotein capsule. S. pneumoniae is a
    gram-positive cocci that does possess a glycoprotein
    capsule.
    Answer D is incorrect. S. aureus displays a β-
    hemolytic pattern, not an α-hemolytic pattern.
    Answer E is incorrect. Bile solubility is not a
    characteristic property of S. aureus; instead, it
    is a property of S. pneumoniae.

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  • 18. 

    A family who recently emigrated from Romania brings their 7-year-old child to the pediatrician with complaints of conjunctivitis and periorbital swelling. The child has had coughing with a runny nose and high fever for 3 days. Small lesions with blue-white centers are seen in his oral cavity. Which of the following is the most likely cause of this child’s symptoms?

    • A. 

      Diphtheria

    • B. 

      Pertussis

    • C. 

      Roseola

    • D. 

      Rubella

    • E. 

      Rubeola

    Correct Answer
    E. Rubeola
    Explanation
    The correct answer is E. Rubeola, also called
    measles, is a relatively rare illness in the United
    States because of the ubiquity of the measles/
    mumps/rubella (MMR) vaccine. It presents
    with the prodrome described in this patient.
    The rash that spreads from head to toe over a
    3-day period develops 1 or 2 days after the appearance
    of Koplik’s spots, which are red oral
    lesions with blue-white centers.
    Answer A is incorrect. Diphtheria is an illness
    virtually unknown in the United States because
    of the prevalence of the DTaP vaccine.
    It is caused by Corynebacterium diphtheriae
    and is characterized by a membranous pharyngitis.
    Answer B is incorrect. Pertussis, or whooping
    cough, is also rare due to widespread vaccinations.
    It is a respiratory infection of children
    that characteristically produces coughing
    spasms followed by a loud inspiratory whoop.
    Answer C is incorrect. Roseola is a febrile disease
    of very young children that begins with a
    high fever and progresses to a rash similar to
    measles. Infants and young children are most
    at risk. It is believed to be caused by human
    herpesvirus 6.
    Answer D is incorrect. Rubella, also known as
    German measles, is a less severe viral exanthem.
    Many infections are subclinical, but rubella
    can cause severe birth defects when infection
    occurs during the prenatal period.

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  • 19. 

    A 34-year-old woman newly diagnosed with HIV infection is unable to access antiretroviral medications. She is aware that if untreated, her disease will progress and make her susceptible to different infections. She inquires about the natural course her disease may take and the different infections she might acquire. Which of the following conditions is most likely to manifest only when the T cell count drops below 50/mm3?

    • A. 

      Disseminated Mycobacterium aviumintracellulare

    • B. 

      Herpes simplex virus infection

    • C. 

      Herpes zoster infection

    • D. 

      Oral thrush

    • E. 

      Pneumonia due to Pneumocystis jiroveci

    • F. 

      Toxoplasmosis brain lesion

    Correct Answer
    A. Disseminated Mycobacterium aviumintracellulare
    Explanation
    The correct answer is A. Disseminated Mycobacterium
    avium-intracellulare commonly infects
    birds and other animals. It can infect humans
    when their T lymphocyte count is below
    approximately 50/mm3. It presents as a chronic
    wasting illness as the bacteria proliferate
    throughout the body. Prophylaxis is with
    azithromycin. This is the only answer choice
    whose development requires a T lymphocyte
    count

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  • 20. 

    An 8-year-old boy is brought to his pediatrician by his parents because of a fever and a sore throat. On examination, he has tonsillar exudates and swollen, tender anterior cervical nodes. His parents report no history of cough. Gram stain of the tonsillar exudate reveals gram-positive cocci. Which of the following describes the organism most likely causing this patient’s symptoms?

    • A. 

      Catalase-negative, α-hemolytic, optochinresistant

    • B. 

      Catalase-negative, α-hemolytic, optochinsensitive

    • C. 

      Catalase-negative, β-hemolytic, bacitracinresistant

    • D. 

      Catalase-negative, β-hemolytic, bacitracinsensitive

    • E. 

      Catalase-positive, coagulase-negative, novobiocin- resistant

    • F. 

      Catalase-positive, coagulase-negative, novobiocin- sensitive

    Correct Answer
    D. Catalase-negative, β-hemolytic, bacitracinsensitive
    Explanation
    The correct answer is D. This patient has a
    classic presentation (fever, sore throat, anterior
    cervical lymphadenopathy, lack of cough) of
    streptococcal pharyngitis, or strep throat. Strep
    throat is caused by Streptococcus pyogenes, or
    group A streptococcus. Diagnosis is confi rmed
    with a rapid strep test or with throat swab culture.
    The treatment of choice is penicillin. To
    differentiate this organism from other gram-positive organisms, several tests can be performed
    in the lab. For example, when cultured
    on blood agar, S. pyogenes creates a clear halo
    (β-hemolysis) around the colonies due to damage
    of the RBCs. To further differentiate streptococcal
    species, sensitivity to different antibiotics
    is measured. S. pyogenes is sensitive to
    bacitracin.
    Answer A is incorrect. Catalase-negative,
    α-hemolytic, optochin-resistant describes the
    viridans streptococci. Streptococcus mutans is
    associated with the formation of dental caries.
    Answer B is incorrect. Catalase-negative,
    α-hemolytic, optochin-sensitive describes Streptococcus
    pneumoniae. This organism causes
    pneumonia and otitis media. Rates of S. pneumoniae
    meningitis have decreased with the advent
    of the pneumococcal vaccine.
    Answer C is incorrect. Catalase-negative,
    β-hemolytic, bacitracin-resistant describes Streptococcus
    agalactiae, or group B streptococcus. S.
    agalactiae is a signifi cant cause of serious bacterial
    infection in neonates.
    Answer E is incorrect. Catalase-positive, coagulase-
    negative, novobiocin-resistant describes
    Staphylococcus saprophyticus. This is the second
    most common cause of urinary tract infection
    in young, healthy women.
    Answer F is incorrect. Catalase-positive, coagulase-
    negative, novobiocin-sensitive describes
    Staphylococcus epidermidis. Infection with S.
    epidermidis is associated with skin penetration
    by implanted prosthetic devices such as prosthetic
    heart valves, intravenous lines, and intraperitoneal
    catheters.

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