Note the 0.1 to 0.5 cm cysts involving the inner medullary and papillary regions in this kidney. Note that the cortex appears normal. This is medullary sponge kidney (MSK), which is congenital, but most often occurs sporadically without a defined inheritance pattern. It is often bilateral, but incidental and found only on radiologic imaging studies, with an incidence of 0.5 to 1% in adults. MSK may become symptomatic in young adults, with onset of recurrent hematuria and/or urinary tract infection as a consequence of formation of calculi, which develop in 60% of cases. Renal failure is unlikely to occur, but may result from severe pyelonephritis.