Xanthogranulomatous pyelonephritis is a type of chronic pyelonephritis.
A significant number of the inflammatory cells are lipid laden histiocytes and account for the yellow appearance of the gross specimen. Cut section of this specimen shows irregular yellowish areas of renal parenchyma that are related to the calyces. The calyces are are dilated and are lined by irregular necrotic tissue. There is a renal calculus lodged in the renal pelvis at the botton of the photograph. The disease process is essentially infectious.