<prompt> The most common infectious agent associated with chronic pyelonephritis isProteus vulgarisKlebsiella pneumoniaStaphylococcus aureusEscherichia coli  <response>: In most urinary tract infections, the infecting organisms are derived from the patients 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 