## **Core Concept**
The question tests knowledge of side effects associated with antitubercular drugs, specifically the risk of hypothyroidism. Antitubercular drugs can have various side effects due to their mechanism of action and metabolism.

## **Why the Correct Answer is Right**
Ethionamide is known to interfere with thyroid function. It inhibits the synthesis of thyroid hormones, leading to hypothyroidism. This side effect is significant enough to warrant monitoring of thyroid function tests in patients on long-term ethionamide therapy.

## **Why Each Wrong Option is Incorrect**
* **Option A:** Isoniazid is primarily associated with hepatotoxicity, peripheral neuropathy, and others, but not commonly with hypothyroidism directly.
* **Option B:** Rifampicin is known for inducing the cytochrome P450 enzyme system, which can affect the metabolism of many drugs, but it is not directly associated with causing hypothyroidism.
* **Option D:** Pyrazinamide can cause hepatotoxicity and hyperuricemia but is not commonly linked with hypothyroidism.

## **Clinical Pearl / High-Yield Fact**
A key clinical point to remember is that ethionamide can cause hypothyroidism, a side effect that may necessitate thyroid hormone replacement therapy. Monitoring thyroid function is crucial in patients on ethionamide.

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