**Core Concept**
Oral contraceptive pills (OCPs) work by inhibiting the release of gonadotropin-releasing hormone (GnRH) from the hypothalamus, which in turn decreases the production of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) from the pituitary gland. This leads to a decrease in estrogen and progesterone production in the ovaries, preventing ovulation.

**Why the Correct Answer is Right**
The correct answer is rifampicin, a potent inducer of the cytochrome P450 enzyme system. Rifampicin increases the metabolism of ethinyl estradiol and other components of OCPs, leading to decreased plasma levels and reduced efficacy of the contraceptive. This is because rifampicin induces the activity of CYP3A4, a key enzyme responsible for the metabolism of many drugs, including OCPs.

**Why Each Wrong Option is Incorrect**
**Option A:** Phenytoin is an enzyme inducer, but it is not as potent as rifampicin in inducing the metabolism of OCPs.
**Option B:** Griseofulvin is an antifungal medication that can interact with OCPs, but it is not a significant inducer of the cytochrome P450 enzyme system.
**Option C:** Isoniazid is an antitubercular medication that can interact with OCPs, but it is not a significant inducer of the cytochrome P450 enzyme system.

**Clinical Pearl / High-Yield Fact**
Rifampicin is a classic example of a medication that can significantly interact with OCPs, leading to contraceptive failure. Patients taking rifampicin or other enzyme-inducing medications should be advised to use alternative methods of contraception.

**Correct Answer:** C. Rifampicin.