## **Core Concept**
The question tests the interaction between oral contraceptive pills (OCPs) and antitubercular drugs. Certain antitubercular drugs are known to affect the efficacy of OCPs, potentially leading to reduced contraceptive effectiveness.

## **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 the estrogen and progesterone components of OCPs, thereby decreasing their plasma concentrations and effectiveness. This can lead to reduced efficacy of OCPs, potentially causing contraceptive failure.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Isoniazid does not significantly induce the cytochrome P450 system to the extent that it affects OCP metabolism in a clinically significant way compared to rifampicin.
- **Option B:** Pyrazinamide does not have a well-documented interaction with OCPs that would decrease their effectiveness.
- **Option C:** Ethambutol does not have significant effects on the cytochrome P450 system or on OCP efficacy.

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

## **Correct Answer:** . Rifampicin