The disease process is essentially infectious. Proteus is a frequent associated causative organism. Chronic obstruction plays a part in the development of the disorder. Pathogenesis Most cases are due to ascending infections from urine flow abnormalities. Urinary outflow obstruction results in urine stasis providing an environment for bacterial proliferation.
Obstruction also predisposes to vesicoureteral reflux which can enable bacteria to reach the kidney.
Xanthogranulomatous pyelonephritis is usually unilateral and may involve all or portion of a kidney. The affected renal parenchymal is brightly yellow and may take on a nodular quality mimicking renal cell carcinoma. Not uncommonly there is associated evidence of urine outflow obstruction such as stone disease and hydronephrosis. The characteristic histologic feature of xanthogranulomatous pyelonephritis is the abundance of lipid-laden macrophages as part of the inflammatory process resulting in the gross yellow appearance. Other components of the inflammation include lymphocytes, plasma cells neutrophils and scattered multinucleated giant cells.