This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Ophthalmology
[{'content': 'The commonest cause of rubeosis iridis is \nSelect and explain. \nA. Diabetes mellitus, B. Central retinal vein occlusion, C. Central retinal aery occlusion, D. Carotid stenosis', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Ans. Diabetes mellitus', 'role': 'assistant'}]