## **Core Concept**
The question tests knowledge of side effects associated with anti-tuberculosis (TB) drugs, specifically the adverse effect of optic neuritis. Optic neuritis is an inflammation of the optic nerve that can lead to acute vision loss. It is a known side effect of certain medications, including some used in the treatment of tuberculosis.

## **Why the Correct Answer is Right**
Ethambutol is the correct answer because it is well-documented to cause optic neuritis as a side effect. This condition can manifest as decreased visual acuity, visual field defects, and even color vision disturbances. The mechanism behind ethambutol-induced optic neuritis is not entirely clear, but it is thought to be related to the drug's toxic effect on the optic nerve. The risk of this side effect is dose-dependent and can be minimized by using the lowest effective dose and monitoring patients' visual acuity.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Isoniazid can cause a variety of side effects, including peripheral neuropathy, hepatotoxicity, and rash, but it is not commonly associated with optic neuritis.
- **Option B:** Rifampicin is known for its potential to cause hepatotoxicity and interact with many other drugs, but optic neuritis is not a commonly reported side effect.
- **Option C:** Pyrazinamide can cause hepatotoxicity and hyperuricemia but is not typically associated with optic neuritis.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that when using ethambutol, especially at higher doses or for prolonged periods, monitoring of visual acuity and color vision is recommended to early detect any signs of optic neuritis. This monitoring is crucial for preventing permanent vision loss.

## **Correct Answer:** . Ethambutol