## **Core Concept**
The question tests knowledge of side effects associated with anti-tuberculosis (TB) drugs, specifically those that cause ocular problems. Anti-TB drugs can have various side effects, and it's crucial to identify them to ensure safe treatment.

## **Why the Correct Answer is Right**
Ethambutol is known to cause optic neuritis, leading to visual disturbances, including decreased visual acuity and even blindness. This side effect is dose-dependent and can be irreversible. The mechanism involves damage to the optic nerve, which can be related to the drug's effect on mitochondrial function and ATP production in the retinal ganglion cells.

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

## **Clinical Pearl / High-Yield Fact**
A crucial point to remember is that when using ethambutol, monitoring of visual acuity and color vision is recommended to detect early signs of optic neuritis. The risk increases with higher doses, and the effect can be irreversible.

## **Correct Answer:** C. Ethambutol.