This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Surgery
Sheela, a middle aged diabetic female presented with flank pain and fever. On U/S the kidney was irregular and showed fat density lesion with calculi. The diagnosis is most probably - 
Select and explain. 
A. TB kidney, B. Xanthogranulomatous kidney, C. Chronic pyelonephritis, D. Renal abscess
B: Xanthogranulomatous kidney
Answer: B: Xanthogranulomatous kidney 
Explanation: Xanthogranulomatous pyelonephritis

Xantho means lipid laden foam cells.
Xanthogranulomatous pyelonephritis is a variant of chronic pyelonephritis that characteristically presents in middle aged patients with a poorly functioning kidney (as in diabetic)
On histology there are areas of clear cells that are lipid laiden macrophages.
These areas of lipid laiden macrophage appear as fat density lesion on U/S.
This pyelonephritis is often accompanied by a calculus and proteus infection.