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: Pathology
An important factor for chronic pyelonerphritis is - 
Select and explain. 
A. Obstruction, B. Vesico-ureteral reflux, C. Pelvi-ureteric junction obstruction, D. Catheter induced
B: Vesico-ureteral reflux
Answer: B: Vesico-ureteral reflux 
Explanation: None