This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Contraception, Subject name: Gynaecology & Obstetrics
OCP's are contraindicated in women receiving which of the following drug ? 
Select and explain. 
A. Rifampicin, B. Ethambutol, C. Streptomycin, D. Pyrazinamide
A: Rifampicin
Answer: A: Rifampicin 
Explanation: OCPs are contraindicated in patient on enzyme-inducing drugs like rifampicin, and antiepileptic except sodium valproate. Rifampicin induces cytochrome 450 ,increasing the hepatic hydroxlation of estrogen by which there is a fourfold increase in its metabolism. This causes clearance and deceased effect of estrogen. Reference: Shaw's textbook of Gynaecology 17th edition,page 265.