Cataract in diabetic patients is because of the accumulation of sorbitol in the lens. The enzyme responsible for this is –
Diabetic cataracts are related to the polyol pathway. High glucose levels lead to the conversion of glucose to sorbitol via the enzyme aldose reductase. Sorbitol can't be converted back efficiently because there's a deficiency in sorbitol dehydrogenase in the lens. This accumulation causes osmotic stress, leading to lens swelling and cataract formation.
The correct answer should be aldose reductase. Now, looking at the options (even though they're not provided), the common distractors would be other enzymes in the pathway. For example, sorbitol dehydrogenase is present in other tissues but not in the lens. Glucokinase might be a distractor as it's involved in glucose metabolism but not in this pathway. Hexokinase is another enzyme that phosphorylates glucose, but again, not part of the polyol pathway.
The clinical pearl here is that aldose reductase inhibitors are being researched for preventing diabetic complications, including cataracts. Students should remember that the key enzyme in this context is aldose reductase.
**Core Concept**
Diabetic cataracts result from the polyol pathway activation in hyperglycemia. Excess glucose is reduced to sorbitol by **aldose reductase**, which accumulates in the lens due to limited sorbitol dehydrogenase activity, causing osmotic swelling and cataract formation.
**Why the Correct Answer is Right**
**Aldose reductase** catalyzes the first step of the polyol pathway, converting glucose to sorbitol in the lens. In diabetes, chronic hyperglycemia increases this pathway’s activity. Sorbitol cannot efficiently exit the lens, leading to osmotic stress, lens cell damage, and opacification. This mechanism is distinct from other metabolic pathways (e.g., glycolysis or hexose monophosphate shunt).
**Why Each Wrong Option is Incorrect**
**Option A:** *Glucokinase* phosphorylates glucose in the liver and pancreas but is not involved in the polyol pathway.
**Option B:** *Hexokinase* catalyzes glucose phosphorylation in glycolysis, unrelated to sorbitol accumulation.
**Option C:** *Sorbitol dehydrogenase* converts sorbitol to fructose in other tissues (e.g., kidneys), but its activity is minimal in the lens, allowing sorbitol to accumulate.
**Clinical Pearl / High-Yield Fact**
Aldose reductase inhibitors (e.g., epalrestat) are investigational therapies for preventing diabetic complications like cataracts. Remember: "Sorbitol = aldose reductase’s product → cataracts in diabetes."
**Correct Answer: D. Aldose reductase**