**Core Concept**
Ethambutol is an antitubercular medication that works by inhibiting the synthesis of the mycolic acid component of the Mycobacterium tuberculosis cell wall. Its characteristic toxicity is related to its effect on the optic nerve, leading to visual disturbances.

**Why the Correct Answer is Right**
The characteristic toxicity of ethambutol is due to its ability to cause optic neuritis, leading to visual field defects, loss of color vision, and even complete blindness in severe cases. This occurs because ethambutol inhibits the Na+/K+ ATPase pump in the optic nerve, leading to a decrease in the intracellular concentration of potassium ions and an increase in sodium ions. This disrupts the normal functioning of the optic nerve, resulting in visual disturbances.

**Why Each Wrong Option is Incorrect**
* **Option A:** This is incorrect because the characteristic toxicity of ethambutol is not related to gastrointestinal disturbances.
* **Option B:** This is incorrect because the characteristic toxicity of ethambutol is not related to neurological effects such as seizures or psychosis.
* **Option D:** This is incorrect because the characteristic toxicity of ethambutol is not related to hepatotoxicity.

**Clinical Pearl / High-Yield Fact**
It's essential to monitor patients on ethambutol therapy for visual disturbances, including loss of color vision and visual field defects, as early detection and discontinuation of the medication can prevent irreversible blindness.

**Correct Answer: C. Optic neuritis**