**Core Concept**
Ethambutol is an antitubercular medication used to treat tuberculosis. Its mechanism of action involves the inhibition of arabinosyl transferases, enzymes essential for the synthesis of the mycobacterial cell wall. Ethambutol's use is associated with potential side effects due to its effects on the optic nerve.

**Why the Correct Answer is Right**
Ethambutol is particularly toxic to the optic nerve in children, causing visual disturbances and potentially leading to permanent vision loss. The exact mechanism is not fully understood, but it is thought to involve the inhibition of the optic nerve's ability to regenerate. **Deficiency of vitamin B6 (pyridoxine)** has been implicated in the pathogenesis of ethambutol-induced optic neuritis.

**Why Each Wrong Option is Incorrect**
* **Option A:** Optic neuritis is not typically associated with ethambutol use, though it can occur with isoniazid or other medications.
* **Option B:** Ethambutol's use is not primarily associated with gastrointestinal side effects like nausea or vomiting.
* **Option C:** Hepatotoxicity is a known side effect of many antitubercular medications, but it is not the primary reason ethambutol is avoided in children.

**Clinical Pearl / High-Yield Fact**
Ethambutol-induced optic neuritis can be prevented or reversed with pyridoxine supplementation in some cases, highlighting the importance of monitoring visual acuity in patients taking this medication.

**Correct Answer: D. Optic neuritis.**