**Core Concept**
Ethionamide is an oral prodrug used in the treatment of multidrug-resistant tuberculosis (MDR TB). It acts by inhibiting the synthesis of mycolic acid, a key component of the Mycobacterium tuberculosis cell wall. Ethionamide's mechanism of action is similar to that of isoniazid, but it has a broader spectrum of activity against M. tuberculosis.

**Why the Correct Answer is Right**
Ethionamide is known to cause thyroid dysfunction, particularly hypothyroidism, as a side effect. This is due to its ability to inhibit the synthesis of thyroid hormones. Ethionamide can interfere with the production of thyroxine (T4) and triiodothyronine (T3) by inhibiting the enzyme 5'-deiodinase, which is essential for the conversion of T4 to T3. As a result, patients treated with ethionamide may experience symptoms of hypothyroidism, such as fatigue, weight gain, and cold intolerance.

**Why Each Wrong Option is Incorrect**
**Option A:** Kanamycin is an aminoglycoside antibiotic that is used to treat MDR TB. While kanamycin can cause ototoxicity and nephrotoxicity, it is not associated with hypothyroidism.
**Option B:** Pyrazinamide is a key component of the first-line treatment regimen for TB and is not known to cause hypothyroidism as a side effect.
**Option D:** This option is incorrect because ethionamide is indeed a drug used to treat MDR TB that can cause hypothyroidism.

**Clinical Pearl / High-Yield Fact**
When treating MDR TB with ethionamide, it is essential to monitor thyroid function regularly to prevent hypothyroidism. Patients may require thyroid hormone replacement therapy if hypothyroidism develops.

**✓ Correct Answer: C. Ethionamide**