**Core Concept**
Ethambutol is a first-line antitubercular medication used to treat tuberculosis (TB). It works by inhibiting the synthesis of mycolic acid, a critical component of the Mycobacterium tuberculosis cell wall. Ethambutol's mechanism of action is unique among antitubercular agents.

**Why the Correct Answer is Right**
Ethambutol's primary concern in children is its potential to cause optic neuritis, leading to visual impairment. This occurs due to its toxic effect on the optic nerve, which can result in decreased visual acuity, color vision defects, or even permanent blindness. The risk of optic neuritis is higher in children due to their developing optic nerves.

**Why Each Wrong Option is Incorrect**
**Option A:** While ethambutol can cause gastrointestinal side effects, such as nausea and vomiting, these are not the primary reasons for avoiding its use in children.
**Option B:** Ethambutol does not have a significant impact on the liver function tests, making liver toxicity an unlikely reason for its avoidance in children.
**Option C:** Although ethambutol can cause peripheral neuropathy in some cases, this is not the primary concern when using the medication in children.

**Clinical Pearl / High-Yield Fact**
Remember the acronym "EIEIO" to associate ethambutol with its potential side effects: **E**ye toxicity, **I**ncreased INR (due to its effect on vitamin K metabolism), **E**levated liver enzymes, **I**ncreased risk of ototoxicity, and **O**ptic neuritis.

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