This image illustrates a horseshoe kidney from an elderly man that was being evaluated for chronic kidney disease associated with diabetes. Horseshoe kidney is the most common renal fusion abnormality, with an incidence of 1/400 worldwide and a male-to-female ratio of 2:1 and is not a cause of impaired kidney function. One theory of the abnormal fusion of the kidneys is that the metanephric blastemas close together before renal capsule maturation, resulting in fusion. The normal embryonic ascent of the fused kidney is disrupted by the inferior mesenteric artery, leaving the adult horseshoe kidney in a lower position than normal. About 90% of the time, the lower poles are fused and the excretory renal units and ureters are maintained separately.