**Core Concept**
Thioacetazone is a first-line antitubercular medication used in the treatment of tuberculosis (TB), particularly in patients with HIV co-infection. However, it is known to cause severe cutaneous adverse reactions, including Stevens-Johnson syndrome (SJS) and toxic epidermal necrolysis (TEN), in susceptible individuals.

**Why the Correct Answer is Right**
Thioacetazone is a prodrug that is activated by the enzyme dihydrofolate reductase (DHFR) in the liver. It is then converted to its active metabolite, which is responsible for its antitubercular activity. In HIV-infected individuals, the compromised immune system and altered drug metabolism may increase the risk of SJS and TEN. The exact mechanism of thioacetazone-induced SJS is not fully understood, but it is thought to involve an abnormal immune response, including the activation of T cells and the release of cytokines.

**Why Each Wrong Option is Incorrect**
**Option A:** Paraaminosalicylate is another antitubercular medication that is used in combination with isoniazid. While it can cause gastrointestinal side effects, it is not commonly associated with SJS and TEN.
**Option B:** Cycloserine is an antibiotic used in the treatment of TB, but it is not typically linked to severe cutaneous adverse reactions like SJS and TEN.
**Option D:** Rifampicin is a key antitubercular medication that is used in combination with other drugs. While it can cause hepatotoxicity and other side effects, it is not commonly associated with SJS and TEN.

**Clinical Pearl / High-Yield Fact**
When treating TB in HIV-infected individuals, it is essential to carefully monitor for signs of SJS and TEN, particularly in patients receiving thioacetazone. Prompt recognition and discontinuation of the offending medication can significantly improve outcomes.

**Correct Answer Line**
✓ Correct Answer: C. Thioacetazone