A middle-aged woman with back and flank pain.

Xanthogranulomatous pyelonephritis is an uncommon variant of chronic pyelonephritis that predominantly affects middle-aged women. Patients usually present with fever, back or flank pain, flank mass, and the constitutional symptoms of fatigue, malaise, weight loss, and anorexia. Rarely, they may present with a draining sinus. There is usually a history of urinary tract infection, obstruction, or instrumentation. Other abnormalities include anemia, leukocytosis, abnormal liver enzymes, pyuria, and hematuria. Mild azotemia may be present, but frank renal failure is rare. Urine and renal tissue cultures are frequently positive. The most commonly isolated bacterial pathogens are P. mirabilis and E. coli, but other organisms have also been implicated. A CT scan is the best radiologic imaging technique to discover the extent of inflammation as well as any involvement of adjacent structures. Lipid-laden macrophages called xanthoma cells characterize the disease at the microscopic level. Nephrectomy is curative. Careful preoperative evaluation will guide surgical planning in choosing an approach that provides adequate exposure of the affected tissue and facilitates subsequent care of the patient.