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?
- A)Irreversible inhibition of DNA gyrase
- B)Reduced renal clearance of ethinyl estradiol
- C)Increased hepatic metabolism due to cytochrome P450 inductionGABARITO
- D)Competitive antagonism at estrogen receptors
- E)Decreased drug distribution from low plasma protein binding
Explicação
Rifampin is a strong cytochrome P450 inducer and accelerates metabolism of many drugs, including oral contraceptives. Faster hepatic clearance lowers hormone levels below the threshold needed to suppress ovulation. Phenytoin can also induce hepatic enzymes, re... Ver explicação completa e trilha adaptativa →