**Core Concept**
Diabetic retinopathy is a microvascular complication of diabetes mellitus, characterized by damage to the blood vessels in the retina. The degree of diabetic retinopathy is influenced by several factors, including the duration of diabetes, blood glucose control, and hypertension.

**Why the Correct Answer is Right**
The degree of diabetic retinopathy is directly related to the duration of diabetes. As diabetes duration increases, the risk of developing retinopathy also increases. This is because prolonged exposure to high glucose levels leads to increased production of advanced glycosylation end-products (AGEs), which accumulate in the retinal tissue and cause vascular damage. Furthermore, the longer duration of diabetes, the more time the retina has to be exposed to the harmful effects of high glucose levels, leading to more severe retinopathy.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because while blood glucose control is an important factor in the development and progression of diabetic retinopathy, it is not the primary factor determining the degree of retinopathy.

**Option B:** This option is incorrect because hypertension is a significant risk factor for diabetic retinopathy, but it is not the primary factor determining the degree of retinopathy.

**Option C:** This option is incorrect because while both factors are important in the development and progression of diabetic retinopathy, neither is the primary factor determining the degree of retinopathy.

**Clinical Pearl / High-Yield Fact**
The "ACCORD" study demonstrated that intensive glucose control can reduce the risk of microvascular complications, including diabetic retinopathy, in patients with type 2 diabetes. However, the study also highlighted the importance of careful monitoring and management of blood glucose levels to avoid hypoglycemia.

**Correct Answer: D. Duration of diabetes**