Kaplan USMLE Step 1: What infected this pet shop owner?

If you are preparing for the United States medical license exam® (USMLE®) Step 1, you may want to know which questions are most often missed by test prep candidates. Check out this example from Kaplan Medical and read an expert explanation of the answer. Also discover all the articles in this series.

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A 45-year-old man presents to the emergency room with a headache, nausea and stiff neck. The patient reports having had flu-like symptoms, nausea and weakness with fever, which lasted two weeks before his current condition. He has no medical history.

The patient owns a pet store specializing in the breeding and sale of hamsters and mice. Her temperature is 38.3 ° C (101 ° F), pulse is 110 beats per minute, blood pressure is 108/65 mmHg, breaths are 20 breaths per minute, and O2 saturation is 93 % in ambient air. The physical examination shows that the patient appears drowsy, has positive Kernig and Brudzinski signs, intact deep tendon reflexes, and generalized muscle weakness.

Laboratory studies show:

  • Alanine aminotransferase (ALT, SGPT): 52 U / L.
  • Aspartate aminotransferase (AST, SCOT): 68 U / L.
  • Platelet count, blood: 98,000 / mm3.
  • Number of leukocytes: 3800 / mm3.

Analysis of the cerebrospinal fluid shows:

  • Number of cells: 18 / mm3.
  • Glucose: 38 mg / dL.
  • Pressure: 190 mm H2O.
  • Protein: 58 mg / dL.

This patient’s clinical picture is best explained by infection with which of the following agents?

A. Cytomegalovirus.

B. Hantavirus.

vs. Lymphocytic choriomeningitis virus.

D. Powassan virus.

E. Rabies virus.

F. Western equine encephalitis virus.

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The correct answer is C.

The patient has signs and symptoms of lymphocytic choriomeningitis (MCL). LCM is a viral infectious disease transmitted by rodents caused by the lymphocytic choriomeningitis virus (LCMV), a member of the family Arenaviridae. The main host of LCMV is the house mouse. Other types of rodents such as hamsters (not natural reservoirs) can become infected with LCMV from wild mice in the breeder, pet store, or home environment.

Symptoms usually appear eight to 13 days after exposure to the virus in biphasic febrile illness. The initial phase, which can last up to a week, usually begins with one or all of the following symptoms: fever, malaise, lack of appetite, muscle pain, headache, nausea and vomiting. After a few days of recovery, a second phase of the disease may occur.

Symptoms may consist of meningitis (eg fever, headache, stiff neck), encephalitis (drowsiness, confusion, sensory disturbances or motor abnormalities such as paralysis) or meningoencephalitis. It is also known to cause acute hydrocephalus.

During the first phase of the disease, the most common laboratory abnormalities are leukopenia and thrombocytopenia. Liver enzymes in serum may also be slightly elevated. After the onset of neurological disease in the second phase, an increase in protein levels, an increase in the number of white blood cells, or a decrease in glucose levels in the cerebrospinal fluid are usually observed.

Aseptic meningitis, encephalitis or meningoencephalitis requires hospitalization and supportive treatment depending on the severity. Although studies have shown ribavirin to be effective against LCMV in vitro, there is no established evidence to support its routine use for the treatment of LCM in humans.

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Kaplan USMLE Preparation Step 1: Modification of Serologic Results

VSchoice A: Cytomegalovirus (CMV) is not transmitted by rodents; it has a human reservoir and is transmitted through saliva, sexual, parenteral or in utero contact. It will cause mononucleosis (monospot negative) in immunocompetent patients, infection in immunocompromised (especially pneumonia in transplant patients), esophagitis, AIDS retinitis (“sightomegalovirus”) and congenital CMV.

VSchoice B: The hantavirus is transmitted through rodent feces, but it will not cause meningitis. Patients with hantaviruses present with pulmonary syndrome (cough, myalgia, dyspnea, tachycardia, pulmonary edema and effusion) and hypotension.

VSchoice D: The Powassan virus is spread to humans through the bite of an infected woodchuck, squirrel or deer tick, not through exposure to rodents. Many people infected with the Powassan virus are asymptomatic. For those who develop symptoms, the time between the tick bite and feeling unwell varies from a week to a month. The first symptoms can include fever, headache, vomiting, and weakness. The Powassan virus can cause serious illness, including encephalitis or meningitis. Symptoms of serious illness include confusion, loss of coordination, difficulty speaking, and seizures.

Choice E: In the United States, infection with the rabies virus is more often caused by bites from bats, racoons and skunks than by bites from dogs; aerosol transmission (eg, bat caves) is also possible. Rodents are very rarely infected with rabies and are not known to transmit rabies to humans.

Choice F: Western equine encephalitis virus is an arbovirus of the genus Alphavirus, family Togaviridae, which circulates in North America between birds and mosquitoes, occasionally causing disease in humans and horses.

Symptoms develop between five and 15 days after being bitten by infected mosquitoes. Most people are asymptomatic or develop mild symptoms, characterized by fever and headache. Symptoms of a serious illness can include headache, high fever, stiff neck, confusion, coma, tremors, seizures, and paralysis. The risk of serious illness increases in older people.

  • The patient shows signs of lymphocytic choriomeningitis; a viral infectious disease transmitted by rodents caused by the lymphocytic choriomeningitis virus, a member of the Arenaviridae family.
  • LCMV causes biphasic febrile illness. The first phase begins eight to 13 days after exposure to the virus.
  • After a few days of recovery, a second phase of the disease may occur. Symptoms may consist of meningitis, encephalitis, and hydrocephalus.

For more prep questions on USMLE Steps 1, 2, and 3, see the other articles in this series.


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