## **Core Concept**
The question tests knowledge of side effects associated with anti-tubercular drugs, specifically the potential to cause thyroid dysfunction. Anti-tubercular drugs can have various side effects, and their impact on thyroid function is an important consideration in the management of tuberculosis.

## **Why the Correct Answer is Right**
Ethionamide is known to interfere with thyroid function. It can cause **hypothyroidism** as a side effect by inhibiting the synthesis of thyroid hormones. This effect is thought to be due to its structural similarity to thiocyanate, which is known to affect thyroid function. Ethionamide can lead to decreased production of thyroid hormones, resulting in hypothyroidism.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Isoniazid can cause peripheral neuropathy, hepatitis, and other side effects but is not commonly associated with causing hypothyroidism directly.
- **Option B:** Rifampicin is known to induce the cytochrome P450 enzyme system, which can affect the metabolism of many drugs, including thyroid hormone replacement therapy, but it is not typically associated with causing hypothyroidism directly.
- **Option D:** Pyrazinamide's common side effects include hepatotoxicity and arthralgias but not typically hypothyroidism.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that **ethionamide** can cause hypothyroidism, and monitoring of thyroid function is recommended in patients on this medication. This is a critical consideration in the management of patients with tuberculosis who are on this drug.

## **Correct Answer:** C. Ethionamide.