**Question:** Which of the following antitubercular drug is not hepatotoxic:

A. Rifampicin
B. Ethambutol
C. Pyrazinamide
D. Isoniazid

**Core Concept:** Hepatotoxicity is a side effect of certain medications that can damage the liver. In the context of antitubercular drugs, this refers to the potential for these medications to cause liver dysfunction or liver failure when used to treat tuberculosis (TB).

**Why the Correct Answer is Right:** Isoniazid (INH) is a widely used antitubercular drug but is generally considered less hepatotoxic compared to other drugs in the regimen. While liver function tests may be elevated during INH therapy, the risk of severe liver injury is relatively low. This is in contrast to other drugs like Rifampicin, Ethambutol, and Pyrazinamide, which are more likely to cause hepatotoxicity.

**Why Each Wrong Option is Incorrect:**

**Option A (Rifampicin):** Rifampicin is a potent antitubercular drug that has high bactericidal activity against Mycobacterium tuberculosis. However, it is a well-known hepatotoxic agent, which means it can cause liver damage or dysfunction when used in the treatment of TB.

**Option B (Ethambutol):** Ethambutol is a bacteriostatic antitubercular drug that works by inhibiting bacterial protein synthesis. Although less hepatotoxic than Rifampicin, it is still associated with a higher risk of hepatotoxicity compared to Isoniazid.

**Option C (Pyrazinamide):** Pyrazinamide is a bactericidal antitubercular drug that targets the biosynthesis of mycolic acids in Mycobacterium tuberculosis. It is generally considered more hepatotoxic than Isoniazid but less hepatotoxic than Rifampicin and Ethambutol.

**Option D (Isoniazid):** Isoniazid is a first-line antitubercular drug that works by inhibiting the synthesis of mycobacterial pyruvate dehydrogenase complex. It is generally less hepatotoxic than Rifampicin and Ethambutol but more hepatotoxic than Pyrazinamide.

**Clinical Pearl:** Monitoring liver function tests (LFTs) is essential during the treatment of TB. While Isoniazid is generally less hepatotoxic, close monitoring is still recommended, especially in patients with risk factors for liver injury, such as excessive alcohol consumption, underlying liver disease, or concomitant medications that can exacerbate liver toxicity.

**Correct Answer:** D. Isoniazid is the correct answer because it is generally considered less hepatotoxic than Rifampicin, Ethambutol, and Pyrazinamide. Although it can cause liver injury, the risk is generally lower than the other options. Close monitoring of liver function tests is still recommended, especially in patients with risk factors for liver injury.