Differential diagnosis of medullary sponge kidney.

The diagnosis of medullary sponge kidney ( MSK ) is usually made as an incidental finding on pyelogram. The essential abnormality of this condition is a " paintbrush " appearance of the calyces due to the presence of contrast within dilated collecting ducts. Since the radiologic features are similar to those of papillary necrosis, a history of analgesic abuse may make the differential diagnosis difficult. We describe a patient whose urogram strongly suggested MSK but who also had a history of analgesic abuse. Special studies demonstrating completely normal renal function and proven MSK in the patient's mother allowed the diagnosis of medullary sponge kidney. We reviewed the similarities and differences between MSK and papillary necrosis to identify features of diagnostic significance.