Antimicrobials USMLE Step 1 Practice Question
A 19-year-old man with bacterial meningitis presents with fever (39.2°C), headache, and neck stiffness. Vital signs show BP 118/76, HR 104, RR 18, SpO2 98% on room air. He receives ceftriaxone 2 g IV every 12 hours. CSF analysis demonstrates adequate antibiotic penetration with ceftriaxone levels of 4.2 mcg/mL. However, head CT imaging reveals purulent meningitis with persistent inflammation. Blood cultures remain negative. Which mechanism best explains inadequate bacterial eradication in the CNS despite adequate drug delivery?
Answer choices
- ACeftriaxone is inactivated by bacterial beta-lactamases in the CSF
- BCeftriaxone achieves only 10-20% CSF levels relative to serum, insufficient bactericidal activityCorrect answer
- CThe patient's immune system is destroying ceftriaxone in the CNS
- DThe blood-brain barrier actively pumps ceftriaxone back into serum
- EMeningitis-causing bacteria are inherently resistant to all third-generation cephalosporins
- FCeftriaxone is rapidly metabolized by hepatic cytochrome P450 enzymes, reducing CNS bioavailability
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