**Core Concept**
PAS (Para-Aminosalicylic Acid) is an antitubercular medication that can cause hypothyroidism as a side effect due to its interference with thyroid hormone synthesis. This is an example of iatrogenic hypothyroidism caused by a medication.

**Why the Correct Answer is Right**
PAS inhibits the enzyme 5'-deiodinase, which is responsible for converting thyroxine (T4) to triiodothyronine (T3), the active form of thyroid hormone. This leads to a decrease in peripheral conversion of T4 to T3, resulting in hypothyroidism. PAS also has a direct toxic effect on the thyroid gland, further contributing to hypothyroidism.

**Why Each Wrong Option is Incorrect**
**Option B:** Ethionamide is another antitubercular medication that can cause gastrointestinal side effects, such as nausea and vomiting, but it is not commonly associated with hypothyroidism.
**Option C:** Cycloserine is an antitubercular medication that can cause neurological side effects, such as seizures and psychosis, but it is not known to cause hypothyroidism.
**Option D:** Pyrazinamide is an antitubercular medication that can cause hepatotoxicity and arthralgias, but it is not associated with hypothyroidism.

**Clinical Pearl / High-Yield Fact**
It is essential to monitor patients taking PAS for signs and symptoms of hypothyroidism, such as fatigue, weight gain, and cold intolerance. Regular thyroid function tests should be performed to detect any abnormalities early.

**✓ Correct Answer: A. PAS**