## **Core Concept**
The question tests knowledge of antitubercular drugs and their side effects, specifically hepatotoxicity. Isoniazid, rifampicin, and pyrazinamide are first-line antitubercular drugs known for their potential to cause liver injury.

## **Why the Correct Answer is Right**
Isoniazid (INH) is a key drug in the treatment of tuberculosis and is well-documented to cause hepatotoxicity. The mechanism involves the formation of toxic metabolites that can lead to liver cell necrosis. Isoniazid-induced hepatotoxicity is more common in individuals with pre-existing liver disease, those over 35 years old, and those taking higher doses or having a history of alcohol abuse. Monitoring of liver function tests is recommended before and during treatment with isoniazid.

## **Why Each Wrong Option is Incorrect**
* **Option A:** Although ethambutol can have side effects like optic neuritis, it is not commonly associated with hepatotoxicity compared to isoniazid.
* **Option B:** Streptomycin, an aminoglycoside antibiotic used in TB treatment, is primarily associated with nephrotoxicity and ototoxicity, not hepatotoxicity.
* **Option D:** Ethionamide is another second-line drug for TB and can cause gastrointestinal disturbances and hepatotoxicity, but it is not the most commonly associated drug with hepatotoxicity compared to isoniazid.

## **Clinical Pearl / High-Yield Fact**
A crucial point to remember is that the risk of hepatotoxicity increases when isoniazid is used in combination with rifampicin and pyrazinamide. Clinicians should monitor liver function tests in patients on these drugs, especially in the initial phase of treatment.

## **Correct Answer:** . Isoniazid