A 20-year-old woman presents with a history of rapid loss of vision in one eye. Examination reveals pain on movement of the eyeball. The appearance of the fundi is normal, but the afferent pupillary response is diminished. Perimetry shows a large central scotoma. Which of the following is the most likely diagnosis?

**Core Concept**
The patient's symptoms are suggestive of an intraocular cause of vision loss, specifically affecting the optic nerve. The diminished afferent pupillary response and large central scotoma are indicative of a problem with the optic nerve head.

**Why the Correct Answer is Right**
The patient's symptoms are consistent with optic neuritis, an inflammatory condition affecting the optic nerve. Optic neuritis typically presents with sudden vision loss, pain on eye movement, and a diminished afferent pupillary response. The inflammation of the optic nerve head leads to a large central scotoma on perimetry. The normal appearance of the fundi is due to the fact that optic neuritis does not cause significant retinal damage.

**Why Each Wrong Option is Incorrect**

**Option A:** This option is incorrect because optic atrophy typically presents with a more gradual decline in vision, atrophy of the optic disc, and a more peripheral scotoma.

**Option B:** This option is incorrect because papilledema is characterized by swelling of the optic disc, which is not present in this patient. Papilledema typically presents with a gradual decline in vision and a more diffuse visual field defect.

**Option C:** This option is incorrect because retinal detachment typically presents with a more sudden onset of visual loss, flashes of light, and floaters. The visual field defect in retinal detachment is typically more peripheral and crescent-shaped.

**Option D:** This option is incorrect because macular degeneration typically presents with a more gradual decline in central vision, typically affecting older adults.

**Clinical Pearl / High-Yield Fact**
Optic neuritis is a common cause of vision loss in young adults and is often associated with multiple sclerosis. The presence of a diminished afferent pupillary response and a large central scotoma should raise suspicion for optic neuritis.

**Correct Answer:** B.

✓ Correct Answer: C. retrobulbar neuritis