Congenital malformation of the terminal collecting ducts near the calyces of the renal pyramids. Malformed ducts dilate, and cysts form in the medulla.

Typically these malformations do not result in any symptomatology, and diagnosis is incidental. Medullary sponge kidney is associated with an increased risk of nephrolithiasis, hematuria, and urinary tract infection.

Prognosis is excellent and typically renal function is preserved.