Figure 4b. Acute pyelonephritis in a 19-year-old woman with acute left flank pain. (a) Initial unenhanced helical CT scan shows an enlarged left kidney with perinephric stranding and urothelial thickening, but no obstructing stone. (b) Contrast-enhanced CT scan shows striated enhancement of the kidney (arrow). Results of urinalysis helped confirm the presence of urinary tract infection.