If you’re preparing for the United States Medical Licensing Examination® (USMLE®) Step 3 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.  

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This month’s stumper

A 68-year-old male is brought to the emergency department for delirium. He has been complaining of lethargy and a dull headache for the last week. This morning, his son noticed a significant decline in the patient's mental status. In addition, there have been involuntary movements of the hands and face. His temperature is 100.9ordmF (38.3ordmC), heart rate is 76 per minute, respiratory rate is 16 per minute, and blood pressure is 140/90 mm Hg. On examination, the patient is disoriented and sleepy. The remainder of the exam is unremarkable. A head CT shows age-related cerebral atrophy. A lumbar puncture reveals 200 RBCs/mL, 130 WBCs/mL (predominantly lymphocytes), cerebrospinal fluid (CSF) glucose of 78 mg/dL, and CSF protein of 180 mg/dL. 

What would be the most appropriate treatment? 

A. Ceftriaxone.

B. Flucytosine.

C. IV acyclovir.

D. IV fluconazole.

E. Valacyclovir.

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The correct answer is C.

Kaplan Medical explains why

The patient's clinical presentation and lab findings are consistent with herpes simplex encephalitis. Acyclovir is the treatment of choice for the reduction of mortality and morbidity from herpes simplex virus encephalitis. Early and aggressive antiviral therapy can prevent mortality and limit the severity of chronic post-encephalitic cognitive impairments. Acyclovir given at 10 mg/kg IV every eight hours is the recommended dose. Trials comparing acyclovir and vidarabine have shown better results with acyclovir.

Why the other answers are wrong

Choice A: Ceftriaxone is the drug of choice for empiric therapy of suspected bacterial meningitis. Bacterial meningitis typically has a higher WBC count in the cerebrospinal fluid with a predominance of neutrophils. Additionally, the CSF protein is higher than 500 mg/dL in patients with bacterial meningitis. The CSF culture is negative in this patient, and antibacterial agents such as ceftriaxone would not be useful. 

Choice B and D: Flucytosine and fluconazole are antifungal agents; neither is useful in viral encephalitis. 

Choice E: A prolonged course of valacyclovir has to be given with the initial treatment of IV acyclovir to prevent relapse but is not recommended as an initial treatment.

Tips to remember

The treatment of choice for herpes simplex encephalitis is IV acyclovir. Oral valacyclovir should be given with the initial course of treatment if it needs to be prolonged to prevent relapse.

For more prep questions on USMLE Steps 1, 2 and 3, view other posts in this series.

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