Supernumerary Kidney With Horseshoe Fusion Anomaly: A Case Report and Comprehensive Review of the Literature

A supernumerary kidney is an extremely rare congenital anomaly of the genitourinary tract characterized by the presence of an accessory kidney with a separate capsule, vascular supply, and collecting system. It may be an isolated anomaly or associated with other congenital anomalies of the genitourinary tract. The fusion of a supernumerary kidney in a horseshoe configuration is a very rare developmental anomaly with few cases reported in the literature. Clinical presentation varies from incidental finding to abdominal pain, urinary tract infection, hydronephrosis, nephrolithiasis, or other complications. We report the case of a 26-year-old man in whom multimodality imaging was performed for renal evaluation in view of increased frequency of micturition. Initial ultrasonography and Doppler evaluation demonstrated anomalous renal morphology with preserved corticomedullary differentiation and no evidence of hemodynamically significant renal artery stenosis. Cross-sectional imaging further delineated the abnormal renal anatomy, confirming the presence of an accessory renal moiety with a fusion anomaly. Renal function and urinary drainage were preserved. No evidence of hydronephrosis or obstructive changes were seen. This case highlights the importance of multimodality imaging in characterizing rare congenital renal anomalies, differentiating a supernumerary kidney from more common anomalies such as duplex kidney or horseshoe kidney, and facilitating accurate diagnosis for clinical management. We also review the embryological basis, imaging characteristics, associated anomalies, and previously reported literature.