Antimicrobials USMLE Step 1 Practice Question
A 41-year-old woman with severe inflammatory acne presents with BP 118/76, HR 82, RR 14, Temp 37.2°C. She is prescribed doxycycline 100 mg orally twice daily and currently takes oral contraceptive and a multivitamin with iron and calcium. After 2 weeks, she reports breakthrough bleeding and denies gastrointestinal symptoms. Serum β-hCG is negative. Which mechanism best explains the reduced contraceptive efficacy in this patient?
Answer choices
- ADoxycycline inhibits estrogen metabolism, reducing hormone levels
- BIron and calcium in the multivitamin chelate doxycycline, reducing absorption
- CDoxycycline disrupts gut flora, impairing enterohepatic circulation of estrogenCorrect answer
- DDoxycycline and estrogen compete for hepatic protein binding
- EOral contraceptives induce doxycycline metabolism, reducing antibiotic efficacy
- FDoxycycline induces hepatic glucuronidation enzymes, accelerating estrogen inactivation
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