## **Core Concept**
The posterior chamber is the space between the iris and the lens (or vitreous in aphakic eyes). In mature cataract, the lens becomes opaque, making direct visualization of the posterior chamber challenging. Ultrasonography is a valuable tool in such cases.

## **Why the Correct Answer is Right**
The correct answer, **B. Ultrasound biomicroscopy (UBM)**, is right because UBM is a high-frequency ultrasound imaging modality that provides detailed images of the anterior segment, including the posterior chamber, even in the presence of opaque media like mature cataract. It helps in assessing the position of the lens, zonules, and ciliary body.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because, although a standard ultrasound can provide some information, it does not offer the detailed resolution of the anterior segment structures that UBM does.
- **Option C:** This option is incorrect as fundoscopy is used for examining the posterior segment of the eye (retina, optic nerve) and is not suitable for examining the posterior chamber in mature cataract.
- **Option D:** This option is incorrect because gonioscopy is used to examine the anterior chamber angle and is not relevant to the examination of the posterior chamber.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that in cases of mature cataract where the view to the posterior segment is obstructed, **ultrasound biomicroscopy (UBM)** is particularly useful for preoperative assessment, especially for evaluating the posterior chamber intraocular lens (IOL) implantation.

## **Correct Answer:** B. Ultrasound biomicroscopy (UBM).