**Core Concept**
During pregnancy, the selection of antitubercular drugs is crucial to ensure the safety of both the mother and the fetus. The main concern is to avoid drugs with potential teratogenic effects or those that may compromise fetal development.

**Why the Correct Answer is Right**
Isoniazid is generally considered safe during pregnancy. However, Ethambutol poses a risk of optic neuritis, which can lead to vision disturbances. This side effect is particularly concerning in pregnant women, as it may affect the fetus's visual development. Pyrazinamide is also considered safe during pregnancy, but Rifampicin can cause hepatotoxicity, which may be exacerbated in pregnant women. Ethionamide is a less commonly used drug, but it can cause gastrointestinal side effects and is not contraindicated during pregnancy.

**Why Each Wrong Option is Incorrect**
**Option A:** Isoniazid is generally considered safe during pregnancy and is often used as part of the standard treatment regimen.
**Option B:** Pyrazinamide is also considered safe during pregnancy and is often used in combination with other antitubercular drugs.
**Option C:** Rifampicin can cause hepatotoxicity, but it is not the most concerning drug in terms of teratogenic effects or fetal development.
**Option D:** Ethionamide is not the most commonly used antitubercular drug during pregnancy, but it is not specifically contraindicated either.

**Clinical Pearl / High-Yield Fact**
When managing tuberculosis in pregnant women, it is essential to use a combination of antitubercular drugs that are safe and effective, while minimizing the risk of side effects to both the mother and the fetus.

**Correct Answer:** C. Rifampicin