**Core Concept**
Glucose and sugar metabolism play a crucial role in the development of cataracts. The excess of certain substances can lead to the formation of advanced glycosylation end-products (AGEs), which accumulate in the lens and cause oxidative stress, leading to cataract formation.

**Why the Correct Answer is Right**
The most relevant substance associated with cataract formation is sorbitol. In the lens, glucose is converted to sorbitol through the action of aldose reductase, an enzyme that is upregulated in diabetes. The accumulation of sorbitol in the lens leads to osmotic changes, causing water to enter the lens and leading to swelling and cataract formation. This process is known as osmotic cataract.

**Why Each Wrong Option is Incorrect**
- **Option A:** While calcium is an essential mineral, its excess is more commonly associated with calcification in other tissues, such as atherosclerosis and kidney stones, rather than cataract formation.
- **Option B:** Oxidative stress is a contributing factor to cataract formation, but it is not the primary substance associated with cataract development.
- **Option C:** Glycogen is an important energy storage molecule, but its excess is not directly linked to cataract formation.

**Clinical Pearl / High-Yield Fact**
Diabetic patients are at a higher risk of developing cataracts due to the accumulation of sorbitol in the lens. Regular monitoring of blood sugar levels and controlling diabetes can help prevent cataract formation.

**Correct Answer: D. Sorbitol.**