- Xanthogranulomatous pyelonephritis is a rare chronic inflammatory condition of the kidney characterized by inflammation with sheets of lipid-laden macrophages, cholesterol clefts and fibrosis. The exact etiology is unclear however the condition is strongly associated with long-standing obstruction with low-grade infection. It typically affects middle aged women but has been seen from ages 2 to 82. Clinical findings include lower urinary tract symptoms, flank pain, a flank mass, fever and weight loss. E coli and Proteus mirabilis are often grown from urine cultures.
- CT findings include a staghorn calculus with contraction of the renal pelvis, multiple rounded hypoechoic foci in the kidney, enlargement of the kidney, and perinephric inflammation or abscesses. The arrangement of multiple rounded hypoechoic foci around the contracted renal pelvis has been described as the bear paw sign and is considered characteristic of the condition.
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