**Question:** A female presented with loss of vision in both eyes and on examination has normal pupillary responses and normal fundus. Her visually evoked response (VER) examination shows extinguished responses. The most likely diagnosis is

A. Optic neuritis
B. Arterial occlusion
C. Central nervous system (CNS) lesion
D. Retinal disease

**Correct Answer:** C. Central nervous system (CNS) lesion

**Core Concept:**

Visually evoked responses (VER) are electrophysiological tests that evaluate the integrity of the visual pathway, from the retina to the visual cortex in the brain. Normal VER results indicate a normal visual pathway, while extinguished responses suggest a problem along the pathway.

**Why the Correct Answer is Right:**

In this case, the patient presents with bilateral loss of vision, normal pupillary responses, and normal fundus examination. These findings are consistent with a central nervous system (CNS) lesion affecting the visual pathway. A CNS lesion can lead to extinguished VER responses due to damage or dysfunction along the visual pathway, which can result in impaired signal transmission from the retina to the brain.

**Why Each Wrong Option is Incorrect:**

A. Optic neuritis (OPT) typically presents with pain on eye movement, visual field defects, and papilledema. Extinguished VER responses are less likely in optic neuritis because the damage is focal to the optic nerve.

B. Arterial occlusion (such as ischemic stroke) can cause visual loss, but normal pupillary responses and normal fundus examination are inconsistent with this diagnosis. Additionally, arterial occlusion usually presents with visual field defects, which are not seen in this case.

D. Retinal disease typically presents with pain on eye movement, scotomata, and a normal fundus examination. Extinguished VER responses are also less likely in retinal disease due to damage being limited to the retina.

**Clinical Pearl:**

In cases of bilateral loss of vision with normal fundus examination and pupillary responses, a CNS lesion should be considered as the most likely cause. VER testing can help differentiate between retinal and optic neuritis, as well as provide insight into the site of the lesion within the CNS. However, normal pupillary responses and fundus examination further narrow down the differential diagnoses to a CNS lesion affecting the visual pathway, such as multiple sclerosis or ischemic stroke involving the brainstem or the thalamus.