Figure 1: (a) Gross nephrectomy specimen showing characteristic lobulated xanthomatous lesions confined to the renal parenchyma (double arrowhead) caused by an obstructing "staghorn" calculus (white arrowhead) within the renal pelvis which has caused secondary hydronephrosis The dilated upper pole calyces contain purulent green colored fluid (single black arrowhead). (b) H and E section showing focal XGN. Note the wedge-shaped area of cortical inflammation (arrowhead) in continuity with a larger nodular area extending into perinephric fat (double arrowhead). (c) Renal parenchymal granulomatous inflammation with multinucleate giant cells and focal necrosis (far right). The inflammatory infiltrate is rich in histiocytes, some of which have a foamy appearance reflecting high intracellular fat content. (d) The inflammatory reaction extends into perinephric fat (represented by the clear spaces) and with time will undergo organization and fibrosis.