**Core Concept**
MIP-26, also known as aquaporin-0 (AQP0), is a protein that plays a crucial role in the structure and function of the lens in the eye. It is a member of the aquaporin family of proteins, which are involved in water transport across cell membranes.

**Why the Correct Answer is Right**
AQP0 is the most abundant protein in the lens and is essential for maintaining the transparency and refractive index of the lens. It facilitates the transport of water across the lens fibers, allowing for proper lens hydration and maintaining the lens's refractive properties. AQP0 is also involved in the regulation of lens metabolism and the prevention of cataract formation.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because MIP-26 is not primarily involved in ion transport across cell membranes.
* **Option B:** This option is incorrect because MIP-26 is not a structural component of the lens capsule.
* **Option C:** This option is incorrect because MIP-26 is not directly involved in the regulation of lens growth and development.

**Clinical Pearl / High-Yield Fact**
AQP0 is unique among aquaporins in that it has a high water permeability and is also a structural component of the lens, making it essential for maintaining lens transparency and refractive index.

**Correct Answer: C. Maintaining lens transparency and refractive index.**