## **Core Concept**
Gonioscopy is a diagnostic technique used in ophthalmology to examine the **anterior chamber angle**, which is the area where the iris and cornea meet. This region is crucial for the drainage of aqueous humor from the eye. The procedure helps in assessing the angle between the iris and the cornea, which is vital for diagnosing and managing glaucoma.

## **Why the Correct Answer is Right**
The correct answer, **anterior chamber angle**, is the primary site of interest in gonioscopy. This technique involves placing a special lens (gonioscope) against the cornea to allow visualization of the angle. The anterior chamber angle is classified based on its openness, which can be crucial for determining the type of glaucoma (e.g., open-angle vs. angle-closure glaucoma) and guiding treatment.

## **Why Each Wrong Option is Incorrect**
- **Option A:** The **lens** is not directly examined through gonioscopy. While the lens can be involved in certain types of glaucoma (e.g., phacodonesis in trauma), gonioscopy specifically targets the anterior chamber angle.
- **Option B:** The **retina** is not studied through gonioscopy. The retina is examined through other ophthalmic procedures like fundoscopy.
- **Option C:** The **vitreous** is also not the focus of gonioscopy. Vitreous examination usually requires ultrasound or other specialized imaging techniques.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that gonioscopy is essential for differentiating between **open-angle** and **angle-closure glaucoma**. This differentiation is critical because the treatment approaches for these two types of glaucoma can be quite different. For instance, acute angle-closure glaucoma is a medical emergency requiring immediate intervention to reduce intraocular pressure.

## **Correct Answer:** D. anterior chamber angle.