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 earliest symptom to occur in corneal ulcer is \nSelect and explain. \nA. Pain, B. Photophobia, C. Loss of sensation, D. Diminished vision', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Ans. Pain', 'role': 'assistant'}]