This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Pharmacology
[{'content': 'A 30-year-old mother of 2 children weighing 60 kg was taking combined oral contraceptive\npill containing levonorgestrel 0.15 mg + ethinylestradiol 30 μg per day cyclically (3 weeks treatment—\n1 week gap). She developed fever with cough and was diagnosed as a case of pulmonary tuberculosis\nafter  sputum  smear  examination.  She  was  put  on  isoniazid  (300  mg)  +  rifampin  (600  mg)  +\npyrazinamide  (1.5  g)  +  ethambutol  (1.0  g)  daily  for  2  months,  followed  by  isoniazid  (600  mg)\n+  rifampin  (600  mg)  thrice  weekly.  In  the  3rd  month,  she  failed  to  have  the  usual  withdrawal\nbleeding during the gap period of contraceptive cycle. After 10 days, her urinary pregnancy test\nwas  found  to  be  positive.What  could  be  the  reason  for  failure  of  the  oral  contraceptive? \nSelect and explain. \nA. Rifampicin is enzyme inducer and induced metabolism of contraceptives, B. Ethambutol is enzyme inducer and induce metabolism of contraceptives, C. Both, D. None', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Rifampin\xa0 is\xa0 known\xa0 to\xa0 induce\xa0 the\xa0 metabolism\xa0 of\xa0 contraceptive\xa0 steroids.\xa0 Thus,\xa0 after\xa0 regular\xa0intake\xa0 of\xa0 rifampin\xa0 for\xa0 more\xa0 than\xa0 2\xa0 weeks\xa0 (needed\xa0 for\xa0 enzyme\xa0 induction)\xa0 the\xa0 steady-state\xa0blood level of levonorgestrel and ethinylestradiol could have fallen below the threshold for\xa0inhibition of ovulation/contraception. As such, fertility was restored and the woman conceived.', 'role': 'assistant'}]