**Core Concept**
Cataracta brunescens, also known as a brown cataract, is a type of age-related cataract characterized by the deposition of a pigment in the lens. This pigment causes the lens to become brown, leading to vision impairment. The underlying mechanism involves the accumulation of a specific pigment in the lens fibers.

**Why the Correct Answer is Right**
The correct answer is melanin, which is a pigment produced by the body. In the case of cataracta brunescens, melanin is deposited in the lens fibers due to the breakdown of the lens's natural antioxidant defenses. This leads to the accumulation of melanin, causing the lens to become brown. The melanin deposition is thought to be related to the aging process and may be exacerbated by factors such as UV radiation and oxidative stress.

* **Option A:** Copper is not directly related to the development of cataracta brunescens. While copper is an essential trace element, its deposition in the lens is not associated with this type of cataract.
* **Option B:** Iron is also not directly related to the development of cataracta brunescens. Iron deposition in the lens can lead to a different type of cataract, known as a snowflake cataract.
* **Option D:** Keratin is a protein found in skin, hair, and nails, but it is not associated with the development of cataracta brunescens.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that cataracta brunescens is often associated with aging and can be exacerbated by factors such as UV radiation and oxidative stress. This type of cataract can be treated with surgery, which involves removing the cloudy lens and replacing it with an artificial lens.

✓ Correct Answer: C. Melanin