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: Anatomy
[{'content': 'All of the following are mesodermal in origin except - \nSelect and explain. \nA. Dilators of iris, B. Iris stroma, C. Ciliary body, D. Choroid', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]