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: Surgery
[{'content': 'Not a cause of acute anal pain \nSelect and explain. \nA. Thrombosed hemorrhoids, B. Acute anal fissure, C. Fistula in ano, D. Perianal abscess', 'role': 'user'}, {'content': 'Answer: C \nExplanation: Ans. c. Fistula in ano', 'role': 'assistant'}]