**Core Concept**
The question is testing the student's understanding of optic nerve pathology, specifically the effects of demyelination on the optic nerve. Demyelination is the process of losing the myelin sheath surrounding nerve fibers, leading to impaired nerve conduction.

**Why the Correct Answer is Right**
The presence of a plaque of demyelination in the optic nerve, as shown in Figure A, is characteristic of Multiple Sclerosis (MS). MS is an autoimmune disease that causes demyelination in the central nervous system, including the optic nerve. The direct pupillary reflex is absent due to the damage to the afferent pathway, while the indirect (consensual) response remains normal because the efferent pathway is intact. The optic disc edema is a result of the inflammation and demyelination.

**Why Each Wrong Option is Incorrect**
**Option A:** While optic neuritis can cause similar symptoms, the presence of a plaque of demyelination on imaging is more specific to Multiple Sclerosis.
**Option B:** Retinal detachment would not typically cause a plaque of demyelination in the optic nerve.
**Option C:** Optic nerve sheath meningioma would cause a more gradual onset of symptoms and would not typically show demyelination on imaging.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that Multiple Sclerosis can present with a wide range of neurological symptoms, including optic neuritis, and that the presence of demyelination on imaging can be a key diagnostic clue.

**Correct Answer:** D. Multiple Sclerosis