**Core Concept**
High blood glucose levels can lead to a condition known as diabetic cataract, where the lens in the eye becomes cloudy and affects vision. This occurs due to the accumulation of sorbitol, a polyol compound, in the lens.

**Why the Correct Answer is Right**
The accumulation of sorbitol in the lens is caused by the enzyme aldose reductase, which converts excess glucose into sorbitol. In individuals with diabetes, chronically high glucose levels lead to an increased activity of aldose reductase, resulting in the accumulation of sorbitol in the lens. This accumulation of sorbitol causes the lens to swell, leading to opacification and a decrease in visual acuity. The increased osmotic pressure due to sorbitol accumulation also leads to the influx of water into the lens, further contributing to its clouding.

**Why Each Wrong Option is Incorrect**
**Option B:** Decreased fructose in the lens - Fructose is not directly involved in the pathogenesis of diabetic cataract. The accumulation of sorbitol, not fructose, is the key factor in this condition.

**Option C:** Increased oxidative phosphorylation in the lens - Oxidative phosphorylation is a process that generates energy for the cell through the transfer of electrons. While oxidative stress can contribute to the development of cataracts, it is not directly related to the accumulation of sorbitol in the lens.

**Option D:** Macular degeneration - Macular degeneration is a separate condition that affects the macula, the part of the retina responsible for central vision. It is not directly related to the accumulation of sorbitol in the lens.

**Clinical Pearl / High-Yield Fact**
The accumulation of sorbitol in the lens is a key factor in the development of diabetic cataract. This can be prevented or delayed with the use of aldose reductase inhibitors, which can reduce the activity of the enzyme responsible for converting glucose to sorbitol.

**✓ Correct Answer: A. Increased sorbitol in the lens**