Supernumerary kidney associated with horseshoe kidney: A very rare congenital renal anomaly

Supernumerary kidney is a rare congenital anomaly, and its coexistence with a horseshoe kidney is exceedingly uncommon. We report the case of an 18-year-old female who presented with a 1-month history of non-specific left flank pain. Abdominal ultrasound revealed a left native kidney with mild hydronephrosis and a caudally located accessory kidney fused superiorly to the native kidney and inferiorly to the lower pole of the right kidney, forming a horseshoe configuration. Computed tomography (CT) urography confirmed a small caudal supernumerary kidney contributing to the parenchymal isthmus and showed a malrotated accessory renal pelvis, while the native left and right renal pelvises were normally oriented. Mild hydronephrosis was limited to the native left kidney, with no evidence of obstruction. The patient was managed conservatively with clinical and imaging follow-up. Follow up ultrasound after 2 weeks demonstrated complete resolution of hydronephrosis and the patient remained asymptomatic. This case highlights an exceptionally rare combination of a supernumerary kidney with horseshoe fusion and underscores the critical role of imaging in accurate diagnosis, anatomical delineation and ongoing follow-up.