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: Medicine
The most common infectious agent associated with chronic pyelonephritis is - 
Select and explain. 
A. Proteus vulgaris, B. Klebsiella pneumonia, C. Staphylococcus aureus, D. Escherichia coli
D: Escherichia coli
Answer: D: Escherichia coli 
Explanation: None