**Question:** Confirmatory investigation in retinitis pigmentosa is-

A. Fundus Fluorescein Angiography
B. Optical Coherence Tomography (OCT)
C. Electroretinography (ERG)
D. Fluorescein Angiography and Optical Coherence Tomography (OCT)

**Correct Answer:** C. Electroretinography (ERG)

**Core Concept:**
Retinitis pigmentosa (RP) is a group of inherited retinal diseases that cause progressive deterioration of photoreceptor cells and retinal pigment epithelial cells. The primary goal of any confirmatory investigation in RP is to evaluate the functional integrity of the photoreceptors and retinal neurons.

**Why the Correct Answer is Right:**
Electroretinography (ERG) is the gold standard confirmatory test for RP diagnosis and monitoring. It measures the electrical activity of the retina in response to light stimuli and can differentiate between different types of retinal disorders.

**Why Each Wrong Option is Incorrect:**

A. Fundus Fluorescein Angiography (FFA) is an imaging test that visualizes the retinal circulation and does not assess the functional integrity of the retina. It is often used to rule out other retinal vascular disorders.

B. Optical Coherence Tomography (OCT) is a non-invasive imaging test that provides cross-sectional images of the retinal layers, but it does not assess the functional integrity of the retina. OCT is useful in monitoring RP progression but not in confirming the diagnosis.

D. Combining both FFA and OCT is incorrect because FFA is not a functional test, while OCT is an imaging test.

**Clinical Pearl:**
RP is a complex disorder involving multiple retinal layers, and each of the wrong options focuses on a specific aspect (circulation or anatomy) rather than the functional integrity of the retina. A combination of functional tests is essential in confirming the diagnosis of RP and monitoring the progression of the disease. Electroretinography (ERG) provides this functional information, making it the correct answer.