**Core Concept**
Retinitis pigmentosa (RP) is a group of genetic disorders affecting the retina's rod photoreceptor cells, leading to progressive vision loss. The disease is characterized by degeneration of the retina's outer layers, resulting in night blindness, loss of peripheral vision, and eventually, central vision loss.

**Why the Correct Answer is Right**
Electroretinography (ERG) measures the electrical activity of the retina in response to light. In retinitis pigmentosa, the degeneration of rod photoreceptor cells leads to a reduction in the amplitude of the a-wave, which reflects the activity of the rod cells. Therefore, the ERG is typically abnormal in patients with RP, showing a reduced or absent a-wave. Option D, stating that ERG is normal, is incorrect because it contradicts the expected ERG findings in RP.

**Why Each Wrong Option is Incorrect**
**Option A:** Pigments present. This is true in retinitis pigmentosa, as the degeneration of the retina leads to the accumulation of pigmented debris, which is visible as a characteristic "bone spicule" pattern in the retina.
**Option B:** Pale waxy disc. This is also true in RP, as the optic disc may appear pale and waxy due to the degeneration of the underlying retinal tissue.
**Option C:** Narrowing of vessels. This can be seen in RP, as the degeneration of the retina leads to a reduction in the caliber of the retinal vessels.

**Clinical Pearl / High-Yield Fact**
In retinitis pigmentosa, the ERG is a crucial diagnostic tool, and a normal ERG can help rule out other causes of visual loss. However, it is essential to note that a normal ERG does not exclude other forms of retinal degeneration or other ocular and systemic conditions that may mimic RP.

**✓ Correct Answer: D. ERG – normal**