**Core Concept**
Retinitis pigmentosa (RP) is a group of genetic disorders that lead to progressive vision loss due to degeneration of the retina. The electroretinogram (ERG) is a diagnostic tool used to assess the function of the retina.

**Why the Correct Answer is Right**
ERG is a useful diagnostic aid in retinitis pigmentosa because it can detect the abnormal electrical activity of the retina. In RP, the ERG shows reduced or absent a-wave and b-wave amplitudes, indicating dysfunction of the photoreceptor cells. The a-wave represents the response of the photoreceptor cells to light, while the b-wave represents the response of the bipolar cells to the signal from the photoreceptors. In RP, the photoreceptors degenerate, leading to reduced or absent a-wave amplitudes.

**Why Each Wrong Option is Incorrect**
**Option A:** ERG is not the primary diagnostic tool for RP. While ERG can provide supportive evidence, the diagnosis is primarily based on clinical examination and family history.
**Option B:** Fluorescein angiography is not a diagnostic tool for RP. It is used to visualize the blood vessels of the retina and is useful in diagnosing conditions such as diabetic retinopathy and age-related macular degeneration.
**Option C:** Optical coherence tomography (OCT) can provide information on the structural changes in the retina, but it is not the primary diagnostic tool for RP. OCT can show thinning of the retinal layers and loss of photoreceptor cells in RP.

**Clinical Pearl / High-Yield Fact**
In retinitis pigmentosa, the ERG is typically abnormal, but the degree of abnormality can vary. A normal or near-normal ERG does not rule out the diagnosis, as some patients may have early or mild disease.

**Correct Answer: D**