**Core Concept**
Optic neuritis is an inflammatory condition affecting the optic nerve, leading to sudden, painless loss of vision. It often presents in young adults and can be associated with multiple sclerosis or other autoimmune disorders.

**Why the Correct Answer is Right**
Optic neuritis typically presents with acute, painless loss of vision in one eye, often accompanied by color vision defects and a relative afferent pupillary defect. The condition is caused by inflammation of the optic nerve, which can be due to demyelination, ischemia, or infection. The optic nerve head may appear normal on examination, but visual acuity and color vision are significantly impaired. In some cases, optic neuritis may be a manifestation of an underlying systemic disease, such as multiple sclerosis or sarcoidosis.

**Why Each Wrong Option is Incorrect**

* **Option A:** Retinal detachment typically presents with sudden, painless loss of vision, but it is often accompanied by photopsia (flashing lights) and a "shower of sparks" due to retinal tears. Additionally, retinal detachment often requires a systemic examination to identify underlying causes, such as myopia or trauma.
* **Option B:** Eale's disease is a rare condition characterized by recurrent retinal and vitreal hemorrhages, often presenting with sudden, painless loss of vision. However, it typically affects young adults with a history of recurrent hemorrhages and retinal scarring, which was not mentioned in the case.
* **Option C:** Glaucoma typically presents with progressive, painless loss of vision due to optic nerve damage from increased intraocular pressure. However, glaucoma often requires a family history of the condition, elevated intraocular pressure, or other systemic risk factors, which were not mentioned in the case.

**Clinical Pearl / High-Yield Fact**
Optic neuritis is a classic exam question, and students should remember that it often presents with acute, painless loss of vision in one eye, accompanied by color vision defects and a relative afferent pupillary defect.

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