USMLE USMLE Certifications USMLE-STEP-1 Questions & Answers

• Question 351:

  A 26-year-old management consultant has recently eaten raw oysters on three occasions. The patient has always been healthy, but suddenly developed fatigue, loss of appetite, nausea, vomiting, abdominal pain, and fever. Following a careful evaluation, his doctor informed him that he is suffering from a viral infection. This infection was caused by a small (2030 nm), non-enveloped, single-stranded RNA virus. This virus is most likely which of the following? Diagnosis of the causative agent of the patient's illness can best be made by which of the following means?

  A. a positive Tzanck smear

  B. assays for specific viral cytopathic effects

  C. detection of IgA antibodies against the specific virus

  D. detection of IgM antibodies against the specific virus

  E. detection of Negri bodies in the patient's tissues

  Correct Answer: D

  Section: Microbiology/Immunology Diagnosis of the patient's infectious hepatitis can best be made by the demonstration of a fourfold rise of IgM against hepatitis Avirus. Patients with hepatitis virus A do not show any meaningful rise of IgA (choice C). Hepatitis virus does not induce any pathognomonic cytopathic effects (choice B). The detection of Negri bodies is useful in the diagnosis of rabies, not hepatitis virus A (choice E). A positive Tzanck test is useful for the diagnosis of herpes virus, not hepatitis virus A (choice A).

• Question 352:

  A 26-year-old management consultant has recently eaten raw oysters on three occasions. The patient has always been healthy, but suddenly developed fatigue, loss of appetite, nausea, vomiting, abdominal pain, and fever. Following a careful evaluation, his doctor informed him that he is suffering from a viral infection. This infection was caused by a small (2030 nm), non-enveloped, single-stranded RNA virus. This virus is most likely which of the following?

  A. adenovirus

  B. hepatitis A virus

  C. hepatitis B virus

  D. rhinovirus

  E. rotavirus

  Correct Answer: B

  Section: Microbiology/Immunology Hepatitis virus A is a small (2030 nm), non-enveloped, single-stranded RNA virus. The virus is transmitted via the fecaloral route, usually by eating contaminated foods, such as oysters grown in polluted waters. The symptoms of hepatitis A infection include fatigue, loss of appetite, nausea, vomiting, abdominal pain, fever, and jaundice. The urine excreted from patients with jaundice may be dark and the feces pale. Adenovirus is a non-enveloped, double-stranded, linear DNA virus. It is transmitted by respiratory secretions, and it causes pharyngitis, conjunctivitis, keratoconjunctivitis, pneumonia, and gastroenteritis (choice A). Hepatitis B is an enveloped, partially doublestranded DNA virus with a circular genome. It is transmitted by infected blood or blood products, sexually, or congenitally (choice C). Rhinoviruses are members of the picornaviruses. Rhinoviruses cause the common cold, and are isolated from people with mild upper respiratory tract infections (choice D). Rotavirus is a member of the reoviruses. Its genome is quite unusual in that it is composed of double- stranded RNA consisting of 10 segments. Rotavirus causes diarrhea in children mostly under the age of 6 (choice E).

• Question 353:

  Which of the following is the drug(s) of choice for the treatment of the graduate student's illness?

  A. chloroquine

  B. metronidazole and iodoquinol

  C. stibogluconate

  D. sulfonamide and pyrimethamine

  E. trimethoprim and sulfamethoxazole

  Correct Answer: B

  Section: Microbiology/Immunology The drugs of choice for acute amoebic dysentery are metronidazole and iodoquinol. Chloroquine (choice A) is normally used for the treatment of malaria. Stibogluconate (choice C) is recommended for the treatment of leishmaniasis. Sulfonamide and pyrimethamine (choice D) is used for the treatment of toxoplasmosis. Trimethoprim and sulfamethoxazole (choice E) are the drugs of choice for the treatment of pneumonia caused by P. carinii.

• Question 354:

  The etiological agent of the graduate student's disease is usually transmitted by the ingestion of which of the following?

  A. cysts

  B. gametocytes

  C. larvae

  D. ova

  E. sporozoites

  Correct Answer: A

  Section: Microbiology/Immunology Cysts are the infective form of E. histolytica and are transmitted by ingestion of infected food or drink. The gametocyte (choice B) is the infective stage of malaria parasites for mosquitoes. Larval penetration of the skin is the mode of transmission for hookworms, Strongyloides and Schistosoma (choice C). Ingestion of ova (choice D) is the mode of transmission of such organisms as Enterobius vermicularis, Ascaris lumbricoides, Trichuris trichiura, and others, but not E. histolytica. Sporozoites (choice E) are the infective stage of malaria parasites spread to humans by mosquito bites.

• Question 355:

  A 27-year-old graduate student, upon her return from vacation in a tropical area with poor sanitary conditions, became ill. She experienced lower abdominal pain, colitis, tenesmus, flatulence, and bloody diarrhea caused by an RBCcontaining parasite. Which of the following represents the most likely diagnosis?

  A. acute amoebic dysentery

  B. malaria

  C. toxoplasmosis

  D. trichomoniasis

  E. trypanosomiasis

  Correct Answer: A

  Section: Microbiology/Immunology The most likely diagnosis of the patient's illness is acute amoebic dysentery, because the classical symptomatology of acute amoebic dysentery is lower abdominal pain, cramping, colitis, tenesmus, flatulence, and bloody diarrhea. Malaria is characterized by periodic fever, fatal cerebral episodes, lysis of red blood cells, and nephritis due to immune complex formation, and is transmitted by mosquito bites (choice B). Trypanosomiasis is transmitted by bites of the tsetse fly and the trypanosomes initially cause chancres at the site of the bite. Then the trypanosomes reach the central nervous system, and the patient has lassitude, develops sleeping episodes, and tissue wasting or even death can occur (choice E). Trichomoniasis is limited to the vagina and is associated with a watery, foul smelling, abundant greenishgray discharge (choice D). Toxoplasmosis is a mild influenza-like disease with possible lymph node enlargement, and may be severe in immunocompromised patients. Congenital infections can damage the eyes or brain and may be fatal (choice C).

• Question 356:

  Transovarian passage of an obligate intracellular agent is associated with which of the following rickettsial diseases?

  A. Brill-Zinsser disease

  B. Epidemic typhus

  C. Human monocytic ehlichiosis

  D. Q fever

  E. Rocky Mountain spotted fever

  Correct Answer: E

  Section: Microbiology/Immunology The human pathogens in the family Rickettsiaceae are small bacteria of the genera rickettsiae, coxiella, and ehrlichia. They are obligate intracellular parasites and mostly transmitted to humans by insect vectors (excepting Q fever). Rickettial infections are typically manifested by fever, rashes, and vasculitis (excepting Q fever and the Erhlichioses). Rickettsia rickettsii, the cause of Rocky Mountain spotted fever (choice E) is passed transovarially in the tick vector. Epidemic typhus (choice B) is caused by R. prowazekii and is not transmitted transovarially in its flea host. Brill-Zinsser disease is a recurrence of epidemic typhus (latent infection). Human monocytic ehlichiosis is caused by Ehrlichea chaffeenes (choice C) which is not transmitted transovarially in its deer tick vector. Q fever (choice D) is a respiratory infection in which the etiologic agent (Coxiella burnetii) is transmitted in dust contaminated with rickettsiae from placenta, dried feces, urine, or milk from infected animals. Aerosols from slaughterhouses may also be a source of infection for Q fever.

• Question 357:

  Small, "fried-egg" colonies are produced in agar medium by which of the following pairs of agents?

  A. Acinetobacter and Legionella species

  B. Legionella and Mycoplasma species

  C. Mycoplasma and Acinetobacter species

  D. Mycoplasma and Gardnerella species

  E. Mycoplasma and L-form bacteria

  Correct Answer: E

  Section: Microbiology/Immunology Mycoplasma are cell-wall-free bacteria, as are L-form (L=Lister Institute) bacteria who temporarily lose their cell wall due to antibiotic treatment. Mycoplasma never possesses a cell wall but L-form bacteria will regain their cell walls when the antimicrobial affecting cell wall synthesis is removed. L- form bacteria have been a suggested mechanism for maintenance of some chronic infections, a controversial concept. All other genera (Acinetobacter, Legionella-- choice A, Gardnerella--choice D) form regular colonies on agar medium. Mycoplasma and Lform bacteria have very small colonies on the agar surface and substantial growth down into the agar medium in the colony center, giving the perception of "fried-egg" morphology to the colony.

• Question 358:

  A 33-year-old male who practices veterinary medicine in Fresno (central California) is seen in the emergency room with chills, high fever, persistent unproductive cough, and generalized malaise. Physical examination reveals no evidence of pharyngitis, sinusitis, or otitis media. The patient does not have a rash and vehemently proclaims that he has not been bitten by any vectors. On his 10-acre farm, he raises sheep, chickens, turkeys, ostriches, and emus. Acomplement fixation assay for Coxiella is negative. Given the PE and history, which of the following is the most likely etiologic agent?

  A. C. pneumonia

  B. C. psittaci

  C. C. trachomatis

  D. Ehrlichia canis

  E. Rickettsia typhi

  Correct Answer: B

  Section: Microbiology/Immunology The term "psittacosis" is applied to the human disease acquired from contact with birds. "Ornithosis" is applied to infections associated with all types of domestic and free-living birds. In humans, C. psittaci (choice B) produces a range of clinical manifestations ranging from severe pneumonia and sepsis to mild, inapparent infection. A sudden onset of illness taking the form of influenza or nonbacterial pneumonia in a person exposed to birds is suggestive of psittocosis/ornithosis. Such infections will respond to tetracycline treatment. C. trachomatis (choice C) is transmitted person to person and is associated with ocular, genital, and some respiratory infections. C. pneumonia (choice A) is the newest chlamydia species which usually presents with asymptomatic or mild illnesses. Ehrlichiae (choice D) are small, gram-negative bacilli (similar to rickettsiae) that cause nonspecific disease symptoms, fever, chills, headache, vomiting, weight loss, with few reports of respiratory problems. R. typhi (choice E) causes endemic typhus with symptoms similar to those of choice D. Rickettsiae are usually transmitted by insect vectors.

• Question 359:

  A foul-smelling specimen was obtained from a 26-year-old female with a pelvic abscess. Culture grew both aerobic and anaerobic gramnegative bacteria. Which of the following represent the cultured organisms?

  A. A. israelii and E. coli

  B. B. fragilis and L. monocytogenes

  C. B. fragilis and N. gonorrhoeae

  D. C. difficile and B. fragilis

  E. E. coli and Peptostreptococcus spp.

  Correct Answer: C

  Section: Microbiology/Immunology Medically important infections due to anerobic bacteria are common. The infections are often polymicrobic, being mixed with other anerobes, facultative anerobes, and aerobes. Anerobes are found in all parts of the body including skin, intestinal tract, oral cavity, and urogenital mucosa. Infection usually occurs when anerobes contaminate usually sterile body sites or are heavily inoculated with anerobes. Most of the B. fragilis bacteria contain small amounts of catalase and SOD, allowing them to survive oxygen toxicity. Their usual clinical manifestation is an abscess. Anerobes produce an extremely foul odor from abscess pus or growth on media. In this scenario, choice C could be the proper answer. A. israelii (choice A) is anerobic but not usually involved in intra-abdominal/pelvic abscess formations. L. monocytogenes (choice B) enters the GI tract by ingestion of contaminated food (cheese or vegetables). L. monocytogenes is usually found in intrauterine infections, bacteremias, and CNS infections. C. difficile (choice D) is usually present in small numbers and increases when broad- spectrum antibiotics have been used with the patient. C. difficile produces toxins that act on the intestinal lining, producing severe diarrhea. Peptostreptococci (choice E) are anerobic streptococci, usually found in the intestine. These and E. coli would not likely to be routinely found as etiologic causes of pelvic abscesses.

• Question 360:

  Antibiotic therapy is a critical step in the proper management of many patients infected with various types of bacteria. Infection with which microorganism listed below would not benefit from even prompt antimicrobial treatment?

  A. B. anthracis

  B. C. botulinum

  C. C. difficile

  D. C. perfringens

  E. C. tetani

  Correct Answer: B

  Section: Microbiology/Immunology Botulism is worldwide in distribution and is caused by C. botulinum (choice B). C. botulinum is an anerobic, spore-producing grampositive organism. During the growth of the vegetative form of the organism, toxins (types AG) are liberated into the environment, especially when the bacteria undergo autolysis. Illness in humans is most often caused by ingestion of foods contaminated with the toxin. Botulism toxin is absorbed from the gut and binds to receptors of presynaptic membranes of motor neurons of the peripheral nervous system and cranial nerves. This inhibits the release of acetylcholine, producing a flaccid paralysis. Use of antibiotics would be inappropriate and useless since no living bacteria are involved in this presentation of disease. Use of antitoxins would be the method of choice for intervention. B. anthracis (choice A) causes anthrax by bacterial spores entering small abrasions in oral mucosa and germinating into vegetative bacteria. Antibiotics would be the treatment of choice in a clinical situation, although vaccines are available for domestic animals. The other three choices (C, D, and E) all include clostridial organisms that cause diseases where antibiotic treatment is necessary.