CNS Pharmacology USMLE Step 1 Practice Question
A 31-year-old woman with generalized seizure disorder and pulmonary tuberculosis presents with a positive pregnancy test despite consistent use of combined oral contraceptives for contraception. She has been taking phenytoin 300 mg daily and rifampin 600 mg daily for 2 months. Vital signs are stable (BP 118/76, HR 78, RR 16, Temp 37°C). Serum phenytoin level is 18 mcg/mL (therapeutic range 10-20). Which mechanism best explains contraceptive failure in this patient?
Answer choices
- AIrreversible inhibition of DNA gyrase
- BReduced renal clearance of ethinyl estradiol
- CIncreased hepatic metabolism due to cytochrome P450 inductionCorrect answer
- DCompetitive antagonism at estrogen receptors
- EDecreased drug distribution from low plasma protein binding
- FImpaired gastrointestinal absorption of oral contraceptives due to enterohepatic circulation disruption
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