ABSTRACT
The right and left kidneys are fused in the midline by a parenchymal or fibrous isthmus. During gestation, the metanephric blastema abnormally migrates toward the midline, leading to contact and fusion of the lower poles. The normal ascent of the kidney is arrested by the inferior mesenteric artery at the level of the L3 or L4 vertebra, and the kidney fails to normally rotate. Consequently, the renal pelvis is ventrally placed and the ureters often course over the isthmus.
