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
Cause of persistance of a sinus or fistulae includes? 
Select and explain. 
A. Foreign body, B. Non dependentt drainage, C. Unrelieved Obstruction, D. All of the above
D: All of the above
Answer: D: All of the above 
Explanation: Ans. is `d' i.e., All of the above