Primary carcinoid tumour with ossification masquerading as calyx stone in a horseshoe kidney.

A case of renal carcinoid tumour arising within a teratoma in a horseshoe kidney in a 23-year-old male is reported. The presenting symptom was abdominal pain, and bone tissue within the tumour was initially misinterpreted as a calyx stone, leading to unsuccessful attempts at extracorporeal shock-wave lithotripsia and percutaneous lithotripsia. This unusual tumour is over-represented in patients with horseshoe kidney and other congenital developmental renal defects.