A young lady taking rifampicin and OCP together become pregnant, most probable cause is
**Core Concept**
Rifampicin, a potent inducer of the cytochrome P450 enzyme system, can significantly decrease the efficacy of oral contraceptives (OCPs) by increasing the metabolism of estrogen and progesterone. This interaction can lead to contraceptive failure and pregnancy.
**Why the Correct Answer is Right**
When rifampicin is co-administered with OCPs, it induces the CYP3A4 enzyme, which is responsible for the metabolism of ethinyl estradiol and progestins. This increased metabolism leads to lower plasma concentrations of these hormones, ultimately compromising the contraceptive efficacy of OCPs. The risk of pregnancy is significantly higher in women taking rifampicin and OCPs together.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is not provided.
**Option B:** While other medications can interact with OCPs, rifampicin is a specific and well-documented inducer of CYP3A4, making it the most probable cause of contraceptive failure.
**Option C:** Although other factors can contribute to contraceptive failure, rifampicin is a well-established cause of reduced OCP efficacy.
**Option D:** This option is not provided.
**Clinical Pearl / High-Yield Fact**
It is essential for women taking rifampicin to use alternative or backup contraceptive methods to prevent pregnancy, as the interaction between rifampicin and OCPs can be unpredictable and may lead to contraceptive failure.
**Correct Answer:** B.