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': 'Following are seen in CRA occlusion except – \nSelect and explain. \nA. Gradual loss of vision, B. Headache, C. Sudden loss of vision, D. Retained central vision', 'role': 'user'}, {'content': 'Answer: A \nExplanation: CRAO causes sudden loss of vision (not gradual loss).\nPatient with a patent cilioretinal artery may retain central vision as the macula may be spared (blood supply for the macula often derives from cilioretinal arteries).\nHeadache may be due to many causes, e.g. if the cause is giant cell arteritis patient may present headache, or CRAO has been reported in association with migraine attacks.', 'role': 'assistant'}]