**Core Concept**
Ethambutol is a first-line antitubercular medication used in the treatment of tuberculosis (TB). It works by inhibiting the synthesis of mycolic acid, an essential component of the mycobacterial cell wall. However, like all medications, ethambutol has potential side effects and toxicities.

**Why the Correct Answer is Right**
Ethambutol is known to cause optic neuritis, which can manifest as retrobulbar neuritis. This is due to the drug's ability to cross the blood-brain barrier and affect the optic nerve. The exact mechanism is not fully understood, but it is thought to involve the inhibition of the mitochondrial enzyme NADH dehydrogenase. This leads to a decrease in the synthesis of ATP, resulting in optic nerve damage. The effects can be reversible with discontinuation of the drug, but in severe cases, it can lead to permanent vision loss.

**Why Each Wrong Option is Incorrect**
**Option B:** Deafness is a known side effect of other antitubercular medications, such as streptomycin and kanamycin, but not ethambutol.
**Option C:** Red urine is not a known side effect of ethambutol. However, it can cause other side effects such as gastrointestinal disturbances, headaches, and dizziness.
**Option D:** Peripheral neuritis is a known side effect of isoniazid, another antitubercular medication, but not ethambutol.

**Clinical Pearl / High-Yield Fact**
It's essential to monitor patients on ethambutol for signs of optic neuritis, such as blurred vision, eye pain, and decreased visual acuity. Regular ophthalmological examinations can help detect these side effects early and prevent permanent vision loss.

**✓ Correct Answer: A. Retro bulbar neuritis**