Horseshoe kidney is the most common type of renal fusion anomaly. It is formed by 2 distinct functioning kidneys on each side of the midline, connected by an isthmus (bridge) of functioning renal parenchyma (as in this case) or fibrous tissue that crosses the midline of the body. The fusion is typically between the lower poles (90%). Consequently the inferior poles point medially, which is the reverse of the normal renal axis. The incidence is approximately 1 in 500.

Complications of a horseshoe kidney include:

1. hydronephrosis (due to UPJ obstruction)
2. infection
3. calculi
4. increased incidence of malignancy (Wilms, transitional cell and carcinoid tumors)
5. increased susceptibility to trauma

Case credit: Ian Bickle.