**Core Concept**
Ethambutol, a first-line antitubercular medication, can cause a specific type of visual defect due to its mechanism of action. Ethambutol inhibits the enzyme mycolic acid synthase, which is involved in the synthesis of the mycobacterial cell wall. This inhibition leads to the accumulation of toxic intermediates, ultimately resulting in the death of the bacteria. However, ethambutol also affects the optic nerve, leading to visual disturbances.

**Why the Correct Answer is Right**
Ethambutol is known to cause a condition called optic neuropathy, which is characterized by damage to the optic nerve. This damage can lead to visual disturbances, including blurred vision, loss of color vision, and even complete blindness. The mechanism of action of ethambutol on the optic nerve is not fully understood, but it is thought to involve the inhibition of the enzyme carnitine palmitoyltransferase, which is essential for the proper functioning of the optic nerve.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify the type of eye defect caused by ethambutol. While ethambutol can cause various eye problems, the correct answer is optic neuropathy.
**Option B:** This option is incorrect because it is a side effect of another antitubercular medication, rifampicin, not ethambutol.
**Option C:** This option is incorrect because it is a side effect of a different medication altogether, not ethambutol.

**Clinical Pearl / High-Yield Fact**
It's essential to monitor patients on ethambutol therapy for signs of optic neuropathy, such as blurred vision or loss of color vision. Regular eye exams can help detect this condition early on, and it may be necessary to discontinue ethambutol if the patient experiences any visual disturbances.

**Correct Answer:** A. Optic neuropathy