## **Core Concept**
The question tests knowledge of antitubercular treatment (ATT) drugs and their side effects, specifically those related to visual deterioration. This requires understanding the pharmacology of ATT drugs and their potential toxicities.

## **Why the Correct Answer is Right**
Ethambutol is an ATT drug known to cause optic neuritis, leading to visual deterioration. This side effect is dose-dependent and can result in decreased visual acuity and even color blindness. The mechanism involves damage to the optic nerve, which can be irreversible if not promptly addressed by discontinuing the drug.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Isoniazid is associated with peripheral neuropathy, hepatitis, and other side effects but is not primarily known for causing visual deterioration.
- **Option B:** Rifampicin is known for its hepatotoxicity and can cause a flu-like syndrome but is not commonly associated with visual deterioration.
- **Option D:** Pyrazinamide can cause hepatotoxicity and hyperuricemia but is not typically linked with visual deterioration.

## **Clinical Pearl / High-Yield Fact**
A crucial point to remember is that ethambutol-induced optic neuritis can be prevented or its severity reduced by ensuring the dose does not exceed 15 mg/kg/day and by regular monitoring of visual acuity in patients on long-term therapy.

## **Correct Answer:** . Ethambutol