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

If you’re preparing for the United States Medical Licensing Examination® (USMLE®) Step 1 exam, you might want to know which questions are most often missed by test-prep takers. Check out this example from Kaplan Medical, and read an expert explanation of the answer. Also check out all posts in this series.

A 45-year-old man comes to the emergency department because of a headache, nausea and a stiff neck. The patient reports having had flu-like symptoms, nausea, and weakness with a fever, which lasted for two weeks prior to his present condition. He has no past medical history.

The patient is a pet shop owner that specializes in breeding and selling hamsters and mice. His temperature is 38.3 °C (101 °F), pulse is 110 beats per minute, blood pressure is 108/65 mmHg, respirations are 20 breaths per minute, and O2 saturation is 93% on room air. Physical examination shows 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.
• Leukocyte count: 3,800/mm³.

Analysis of cerebrospinal fluid shows:

• Cell count: 18/mm³.
• Glucose: 38 mg/dL.
• Pressure: 190 mm H₂O.
• Protein: 58 mg/dL.

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

A. Cytomegalovirus.

B. Hantavirus.

C. Lymphocytic choriomeningitis virus.

D. Powassan virus.

E. Rabies virus.

F. Western equine encephalitis virus.

The correct answer is C.

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

The onset of symptoms usually occurs eight to 13 days after exposure to the virus as part of a biphasic febrile illness. The initial phase, which may last as long as a week, typically begins with any or all of the following symptoms: fever, malaise, lack of appetite, muscle aches, headache, nausea, and vomiting. Following a few days of recovery, a second phase of illness may occur.

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

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

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

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