What occurs when developing kidneys fuse in the pelvis and one ascends?

The phenomenon described in the question refers to a condition known as crossed fused ectopia. This occurs when the developing kidneys that are supposed to ascend to their respective positions in the abdominal cavity become fused together while they are still in the pelvis. As one of the kidneys ascends, it carries the other kidney with it, resulting in both kidneys being located on the same side of the body, often in an abnormal position.

In this condition, the fused kidneys are often located on one side of the spine, and instead of having two separate kidneys positioned correctly in the renal fossa, there is a single mass of tissue that may function inadequately. This configuration contrasts with conditions like horseshoe kidney, where the kidneys remain connected but maintain their typical anatomical parlance across the midline, or pelvic kidney where a single kidney does not ascend at all.

Crossed fused ectopic kidney highlights the importance of understanding renal embryology and the various anomalies that can occur. These variations can lead to specific clinical considerations, including the possibility of complications arising from ectopic renal locations, which might predispose individuals to issues like obstruction or urinary tract infections.