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
Not a cause of acute anal pain is: 
Select and explain. 
A. Perianal abscess, B. Thrombosed hemorrhoids, C. Acute anal fissure, D. Fistual in ano
D: Fistual in ano
Answer: D: Fistual in ano 
Explanation: None