**Core Concept**
The maintenance of ascorbate and a-tocopherol in a reduced state in the lens is crucial for preventing oxidative stress and cataract formation. This process involves specific enzymatic mechanisms to recycle these antioxidants. The lens is particularly susceptible to oxidative damage due to its high metabolic rate and limited antioxidant defenses.

**Why the Correct Answer is Right**
The correct answer involves the enzyme **glutathione reductase**. Glutathione reductase is a key enzyme that maintains the balance of glutathione (GSH) in its reduced form (GSH2-) in the lens. Reduced glutathione is essential for recycling ascorbate (vitamin C) and a-tocopherol (vitamin E) from their oxidized forms back to their active, reduced states. This recycling process prevents the accumulation of oxidative damage and maintains the integrity of the lens. The reduced form of glutathione also acts as an electron donor for the regeneration of ascorbate and a-tocopherol, ensuring their continued antioxidant activity.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not relevant to the maintenance of ascorbate and a-tocopherol in the lens.

**Option B:** While glutathione peroxidase is an important enzyme in the lens, it is not directly involved in maintaining ascorbate and a-tocopherol in their reduced states.

**Option C:** This option is incorrect because the enzyme catalase is primarily involved in the decomposition of hydrogen peroxide (H2O2) rather than the recycling of ascorbate and a-tocopherol.

**Clinical Pearl / High-Yield Fact**
Antioxidants such as ascorbate and a-tocopherol play a crucial role in maintaining lens integrity and preventing cataract formation. The recycling of these antioxidants by glutathione reductase is essential for their continued function, and disturbances in this process can contribute to age-related cataracts.

**Correct Answer: D. Glutathione reductase.**