This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Kidney disorders, Subject name: Pathology
Most common infectious agent associated with chronic pyelonephritis is: 
Select and explain. 
A. Proteus vulgaris, B. Klebsiella pneumonia, C. Staphylococcus, D. E.Coli
D: E.Coli
Answer: D: E.Coli 
Explanation: Chronic pyelonephritis: Bacteriological examination of the urine reveals: E.Coli ( most common ) Proteus species Streptococcus faecalis Pseudomonas species Microscopic features: Tubular thyroidization (filled with colloid casts) Interstitial fibrosis and inflammation Obliterative endaeritis of vessels