This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Urinary tract, Subject name: Pathology
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: In most urinary tract infections, the infecting organisms are derived from the patient's own fecal flora. The most common is E.coli, followed by Proteus, Klebsiella, and Enterobacter. Ref: Pathologic basis of disease Robbins, volume 2, Pg no:930