**Core Concept**
The question requires knowledge of the hepatotoxic potential of antitubercular drugs, specifically which one is most likely to cause liver damage. The underlying pharmacological principle being tested is the metabolism and toxicity of these drugs, particularly their effects on liver enzymes and histology.

**Why the Correct Answer is Right**
Isoniazid (INH) is a first-line antitubercular agent that is known for its high hepatotoxic potential. The mechanism of INH-induced liver toxicity involves the formation of reactive intermediates that damage hepatocytes, leading to necrosis and inflammation. This is mediated through the metabolism of INH by cytochrome P450 enzymes, particularly CYP2E1, which converts INH into a reactive acetylhydrazine intermediate that covalently binds to cellular macromolecules, causing cellular damage.

**Why Each Wrong Option is Incorrect**
**Option A:** Rifampicin is also a first-line antitubercular agent, but it is less hepatotoxic than INH. While rifampicin can cause liver enzyme elevations, it is generally well-tolerated and not associated with severe liver damage.

**Option B:** Ethambutol is another first-line antitubercular agent that is primarily associated with optic neuritis and other ocular toxicities, rather than liver toxicity.

**Option C:** Pyrazinamide is a first-line antitubercular agent that can cause liver enzyme elevations and is associated with a higher risk of liver toxicity than INH, but it is not the most hepatotoxic agent among the options.

**Clinical Pearl / High-Yield Fact**
It's essential to monitor liver function tests (LFTs) in patients receiving INH, particularly during the initial months of treatment, as the risk of liver toxicity is highest during this period.

**Correct Answer:** A. Isoniazid