**Core Concept**
Adult-onset retinitis pigmentosa (RP) is a group of inherited disorders characterized by progressive degeneration of the retina, leading to blindness. Cataracts are a common complication in patients with RP, particularly in adults. The most frequent cataract type seen in these patients is related to the underlying pathophysiology of RP.

**Why the Correct Answer is Right**
The correct answer is related to the **sugar cataract**, also known as **sodium cataract** or **diabetic cataract**. In RP patients, the accumulation of **sorbinol dehydrogenase** deficiency leads to the accumulation of **sorbitol** in the lens, causing osmotic changes and ultimately resulting in the formation of a **sugar cataract**. This type of cataract is characterized by a clear nucleus with a central or posterior subcapsular opacity.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not relevant to the context of RP and cataracts.
* **Option B:** This option does not accurately describe the most frequent cataract type seen in adult RP patients.
* **Option D:** This option is not a recognized type of cataract associated with RP.

**Clinical Pearl / High-Yield Fact**
In patients with RP, the accumulation of sorbitol in the lens is a key factor in the development of sugar cataracts. This is due to the deficiency of sorbinol dehydrogenase, an enzyme that breaks down sorbitol.

**Correct Answer: C. Sugar cataract**