**Core Concept**
Ring scotoma is a type of visual field defect characterized by a loss of vision in a ring-shaped pattern, typically affecting the peripheral visual field. This phenomenon occurs due to damage or degeneration of the retinal ganglion cells or the optic nerve.

**Why the Correct Answer is Right**
Retinitis pigmentosa is a group of genetic disorders that affect the retina, leading to progressive vision loss. The condition is characterized by the degeneration of rod photoreceptor cells, which are responsible for peripheral and night vision. As the disease progresses, patients often experience a ring scotoma, where the peripheral visual field is affected, while central vision remains relatively preserved. This occurs due to the relative sparing of cone photoreceptor cells in the central retina.

**Why Each Wrong Option is Incorrect**
**Option A:** Papilledema is characterized by swelling of the optic disc due to increased intracranial pressure, leading to a central or diffuse visual field defect, not a ring scotoma.

**Option B:** Macular edema involves fluid accumulation in the macula, causing a central visual field defect, typically affecting vision in the center of the visual field, not the periphery.

**Option C:** Central retinal artery occlusion (CRAO) causes a sudden, painless loss of vision in one eye, often with a "cherry-red" spot appearance on fundoscopy, but not a ring scotoma.

**Clinical Pearl / High-Yield Fact**
Retinitis pigmentosa often presents with night blindness, followed by progressive loss of peripheral vision, and eventually, central vision. It is essential to recognize the characteristic ring scotoma in patients with retinitis pigmentosa to make an accurate diagnosis.

**✓ Correct Answer: D. Retinitis pigmentosa**