A 55-year-old woman with a long history of recurrent urinary tract infections and dull left flank pain undergoes contrast-enhanced CT of the abdomen. The left kidney is enlarged but maintains its reniform shape. There are multiple rounded low-attenuation areas arranged peripherally around a contracted central collecting system, giving the appearance described as the 'bear paw sign'. No calcification is seen. What is the most likely diagnosis?

• A Renal cell carcinoma
• B Renal lymphoma
• C Renal tuberculosis
• D Xanthogranulomatous pyelonephritis ✓

Explanation

Xanthogranulomatous pyelonephritis results from chronic suppurative infection, usually Proteus or E coli, often with an obstructing stone. Lipid-laden macrophages destroy the parenchyma, producing dilated calyces filled with purulent debris surrounding a scarred contracted pelvis, the bear paw sign on CT. Renal tuberculosis would show calcification, infundibular strictures, or a putty kidney, which are absent here.

Reference: Brant and Helms Fundamentals of Diagnostic Radiology, 4th ed.

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Written and medically reviewed by the StethoPrep medical team.