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Horseshoe kidney is the most common renal fusion anomaly, occurring in approximately 1 in 500 births, where the lower poles of both kidneys are connected by an isthmus of renal tissue or fibrous band. This congenital malformation results from abnormal embryological development during the 4th-6th weeks of gestation when the metanephric blastemas fuse before normal kidney ascent.
The fusion typically occurs at the lower poles and creates a horseshoe-shaped structure that becomes trapped behind the inferior mesenteric artery during normal renal ascent, preventing proper rotation and positioning. This anatomical abnormality often results in malrotation, abnormal vascularization with multiple renal arteries, and altered collecting system drainage that predisposes to urinary stasis and complications.
Most patients with horseshoe kidney remain asymptomatic, but the condition increases risk for urinary tract infections, nephrolithiasis, and hydronephrosis due to ureteropelvic junction obstruction from abnormal insertion angles. Associated conditions include Turner syndrome, trisomy 18, and VACTERL association, making genetic evaluation important in diagnosed patients with additional anomalies.