**Core Concept**
Cotton wool spots are small, white lesions that appear on the retina due to the accumulation of axoplasmic debris and ischemic changes in the retinal nerve fiber layer. They are a hallmark of diabetic retinopathy and other conditions that affect the retina, such as hypertensive retinopathy and retinal vein occlusions.

**Why the Correct Answer is Right**
Cotton wool spots are a result of ischemia and axonal damage in the retinal nerve fiber layer. The damage leads to the leakage of axoplasmic contents into the interstitial tissue, causing the characteristic white appearance. This is often seen in conditions that cause retinal ischemia, such as diabetic retinopathy, hypertensive retinopathy, and retinal vein occlusions.

**Why Each Wrong Option is Incorrect**
**Option A:** Diabetic retinopathy is a common cause of cotton wool spots, so this option is not correct.
**Option B:** Hypertensive retinopathy also causes cotton wool spots due to retinal ischemia, making this option incorrect.
**Option C:** Retinal vein occlusion is another condition that can cause cotton wool spots, so this option is not correct.

**Clinical Pearl / High-Yield Fact**
Cotton wool spots are a sign of retinal ischemia, which can lead to more severe complications such as retinal detachment and vision loss.

**Correct Answer:** D.