## **Core Concept**
The question tests the interaction between anti-tubercular drugs and oral contraceptive pills (OCPs). Certain drugs used in the treatment of tuberculosis can induce liver enzymes, which in turn can affect the metabolism and efficacy of oral contraceptives.

## **Why the Correct Answer is Right**
Rifampicin (also known as rifampin) is a well-known inducer of the cytochrome P450 enzyme system in the liver. This induction increases the metabolism of estrogen and progestin components of oral contraceptives, thereby reducing their plasma concentrations and effectiveness. This can lead to decreased efficacy of OCPs, potentially resulting in contraceptive failure.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Isoniazid is primarily metabolized by the liver and can have hepatotoxic effects but does not significantly induce the cytochrome P450 system to the extent that it affects OCPs in the same way rifampicin does.
- **Option B:** Ethambutol works by inhibiting cell wall synthesis in Mycobacterium tuberculosis and does not have significant effects on the cytochrome P450 system or OCP metabolism.
- **Option C:** Pyrazinamide is another first-line antitubercular drug that can cause hepatotoxicity but does not have a significant effect on the metabolism of oral contraceptives.

## **Clinical Pearl / High-Yield Fact**
A crucial point to remember is that women on OCPs who are started on rifampicin for tuberculosis should be advised to use an additional or alternative method of contraception, such as condoms, to prevent unplanned pregnancy.

## **Correct Answer:** . Rifampicin