Bear's Paw Sign: A Classic Presentation of Xanthogranulomatous Pyelonephritis.

A 41-year-old woman with recurrent urinary tract infections presented with right flank pain, fever, and anorexia. Physical examination revealed right-sided flank pain and a right-sided unilateral renal mass. Computed tomography of the abdomen and pelvis revealed xanthogranulomatous pyelonephritis with perinephric extension. The bear's paw sign was visualized. The classic radiology triad (unilateral large kidney, renal pelvis stone, and a nonfunctioning or poorly functioning kidney) was also found on computed tomographic imaging. She completed a course of antimicrobial therapy and underwent a nephrectomy. Her post-operative course was uneventful and she was discharged from the hospital on postoperative day 4.