**Core Concept**
Rubeosis iridis, also known as neovascularization of the iris, is a condition characterized by the growth of new blood vessels on the surface of the iris. This phenomenon is often associated with chronic ocular diseases, particularly diabetic retinopathy.

**Why the Correct Answer is Right**
The most common cause of rubeosis iridis is diabetic retinopathy. In this condition, the growth of new blood vessels on the iris is a result of the release of angiogenic factors, such as vascular endothelial growth factor (VEGF), which stimulate the proliferation of endothelial cells and the formation of new blood vessels. The increased blood flow and oxygenation of these new vessels can lead to the growth of fibrovascular tissue on the iris surface.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because while retinal detachment can be associated with neovascularization, it is not the most common cause of rubeosis iridis.
* **Option B:** This option is incorrect because while glaucoma is a common ocular condition, it is not directly associated with the growth of new blood vessels on the iris.
* **Option C:** This option is incorrect because while trauma can cause neovascularization, it is not the most common cause of rubeosis iridis.

**Clinical Pearl / High-Yield Fact**
Rubeosis iridis is often a sign of advanced diabetic retinopathy and can lead to complications such as neovascular glaucoma, which is a medical emergency requiring prompt treatment.

**Correct Answer: D. Diabetic retinopathy.**