A child presents with night blindness, delayed dark adaptation. Which investigation is to be done fuher to confirm the diagnosis?
**Core Concept**
Night blindness and delayed dark adaptation are symptoms of a condition known as **nyctalopia**, which is often associated with disorders affecting the **rod photoreceptors** in the retina. These cells are responsible for peripheral and night vision.
**Why the Correct Answer is Right**
The correct investigation to confirm the diagnosis involves assessing the function of the **rod photoreceptors**. The **ERG (Electroretinogram)** measures the electrical activity of the retina in response to light, providing information on the function of rod and cone photoreceptors. In cases of nyctalopia, the rod photoreceptors are often affected, leading to a reduced or absent response on the ERG. This test is essential in diagnosing and differentiating between various retinal disorders, including **retinitis pigmentosa**.
**Why Each Wrong Option is Incorrect**
**Option A:** The **Visual Acuity Test** assesses central vision and is not specific for diagnosing night blindness or nyctalopia. While it may indicate a problem with visual function, it does not provide information on the function of rod photoreceptors.
**Option B:** The **Color Vision Test** evaluates the function of cone photoreceptors, which are responsible for color vision and central vision. This test is not relevant for diagnosing nyctalopia or disorders affecting rod photoreceptors.
**Option C:** The **Tonometry Test** measures intraocular pressure and is used to diagnose and monitor **glaucoma**. While glaucoma can cause vision loss, it is not directly related to night blindness or delayed dark adaptation.
**Option D:** The **Ophthalmoscopy Exam** involves visualizing the retina and is used to diagnose various retinal disorders. However, it is a subjective test and does not provide a quantitative measure of retinal function like the ERG.
**Clinical Pearl / High-Yield Fact**
In cases of nyctalopia, a **family history** of similar symptoms is common, as many disorders affecting rod photoreceptors are inherited in an **autosomal recessive** pattern.
**Correct Answer:** C. Tonometry Test