Solution: Medullary sponge kidney is a disease where there is cystic dilatation of multiple collecting ducts of the medulla. This ultimately leads to medullary nephrocalcinosis, which can lead to nephrolithiasis. Estimated incidence is 1 in 5-20,000. The pathogenesis is unknown, but it is generally sporadic and occasionally inherited. The number and size of renal calcifications often increase over time and complications include infection, renal insufficiency and renal failure, although most patient do quite well. Treatment is only performed if there are complications. The findings on US are consistent with medullary nephrocalcinosis and CT confirms the findings.