## **Core Concept**
The question tests the knowledge of side effects associated with antitubercular drugs, specifically the one causing optic neuritis. Optic neuritis is an inflammation of the optic nerve, which can lead to loss of vision. It is a known side effect of certain medications used in the treatment of tuberculosis.

## **Why the Correct Answer is Right**
Ethambutol is the antitubercular drug known to cause optic neuritis as a side effect. This condition can manifest as decreased visual acuity, visual field defects, or even color blindness. 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 is known to cause peripheral neuropathy, hepatitis, and other side effects but is not commonly associated with optic neuritis.
- **Option B:** Rifampicin can cause hepatotoxicity, gastrointestinal disturbances, and immunological reactions but is not typically linked with optic neuritis.
- **Option C:** Pyrazinamide can cause hepatotoxicity and arthralgias but is not commonly associated with optic neuritis.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that ethambutol-induced optic neuritis is dose-dependent, and regular monitoring of visual acuity is recommended, especially at higher doses. Early detection of visual disturbances can help prevent permanent vision loss.

## **Correct Answer:** D. Ethambutol.