**Core Concept**
Rubeosis iridis refers to the growth of new, fragile blood vessels on the surface of the iris, which is a common complication in patients with diabetes mellitus. This condition is characterized by the formation of a fibrovascular membrane that can lead to vision loss due to neovascularization and subsequent tractional retinal detachment.

**Why the Correct Answer is Right**
The most common cause of rubeosis iridis is diabetic retinopathy, particularly proliferative diabetic retinopathy. In this condition, the prolonged high glucose levels lead to the release of angiogenic factors, such as vascular endothelial growth factor (VEGF), which stimulate the growth of new blood vessels on the iris. The neovascularization is thought to be mediated by the hypoxic environment in the ischemic retina, which triggers a cascade of events leading to the formation of new blood vessels.

**Why Each Wrong Option is Incorrect**
* **Option A:** Cataract is not a direct cause of rubeosis iridis, although it may be a comorbidity in patients with diabetes.
* **Option B:** Hypertension is a risk factor for diabetic retinopathy but is not a direct cause of rubeosis iridis.
* **Option C:** Glaucoma is a separate condition that may be associated with rubeosis iridis, but it is not the primary cause of this condition.

**Clinical Pearl / High-Yield Fact**
Rubeosis iridis is a significant indicator of advanced diabetic retinopathy and requires prompt evaluation and treatment to prevent vision loss. Early detection and management of diabetic retinopathy can help prevent the development of rubeosis iridis.

**Correct Answer:** D. Diabetic retinopathy