**Core Concept**
The sudden, painful loss of vision is a symptom of an emergent condition that requires prompt medical attention. This phenomenon is typically associated with an intraocular or optic nerve pathology.

**Why the Correct Answer is Right**
The most common cause of sudden, painful loss of vision is acute angle-closure glaucoma, also known as narrow-angle glaucoma. This occurs when the iris suddenly blocks the drainage angle of the eye, causing a rapid increase in intraocular pressure (IOP). The increased IOP compresses the optic nerve, leading to sudden vision loss. The pain is due to the increased IOP, which irritates the surrounding ocular tissues.

**Why Each Wrong Option is Incorrect**
* **Option A:** Central retinal artery occlusion typically presents with painless, sudden vision loss in one eye, often due to emboli or vasculitis.
* **Option B:** Optic neuritis typically presents with pain on eye movement and gradual vision loss, often due to inflammation of the optic nerve.
* **Option C:** Macular degeneration typically presents with gradual, painless vision loss in the central visual field.

**Clinical Pearl / High-Yield Fact**
In cases of acute angle-closure glaucoma, treatment typically involves administering intravenous acetazolamide to reduce IOP, followed by surgical iridotomy to prevent future episodes.

**Correct Answer: D. Acute angle-closure glaucoma**