Most cases of acute pyelonephritis are caused by E. coli. The presence of white cell casts is evidence that the infection is present in the kidney not the urinary bladder since the casts are formed in renal tubules. Concomitant cystitis may be present with symptoms of urgency, frequency, and dysuria but acute pyelonephritis may occur without cystitis. In the gross image shown there are numerous small abscesses and the microscopic image of the kidney shows infiltration with white cells.