Antimicrobials USMLE Step 1 Practice Question
A 34-year-old man with HIV infection (CD4 count 175 cells/μL) presents with a 3-week history of productive cough, fever, and night sweats. Chest X-ray reveals bilateral upper lobe infiltrates with cavitation. Sputum smear microscopy is positive for acid-fast bacilli. He is initiated on a standard four-drug tuberculosis regimen (rifampicin, isoniazid, pyrazinamide, and ethambutol) and continues his current efavirenz-based antiretroviral therapy. Laboratory values: ALT 42 U/L, AST 38 U/L, CD4 count 175 cells/μL, viral load 45,000 copies/mL. Vital signs are stable. Two weeks into treatment, plasma efavirenz concentrations fall 40% below the therapeutic range despite consistent medication adherence and normal renal/hepatic function. Which of the following best explains this pharmacokinetic interaction?
Answer choices
- AIsoniazid competitively inhibits CYP2B6, the primary metabolic enzyme for efavirenz
- BPyrazinamide increases gastric pH, reducing efavirenz absorption from the gastrointestinal tract
- CRifampicin induces CYP3A4 and CYP2B6, increasing efavirenz hepatic metabolism and clearanceCorrect answer
- DEthambutol forms a chelation complex with efavirenz, reducing its bioavailability
- ETuberculosis-induced hepatic inflammation impairs efavirenz protein binding and renal excretion
- FRifampicin inhibits the efavirenz transporter p-glycoprotein, decreasing intestinal absorption
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