What is the most common form of renal fusion?

• A. Horseshoe kidney
• B. Crossed fused ectopia
• C. Duplex kidney
• D. Renal agenesis

Answer: (A)

Horseshoe kidney is the most common renal fusion anomaly. It happens when the lower poles of the developing kidneys fuse, forming an isthmus that crosses the midline. As the kidneys normally ascend from the pelvis, the fused mass is halted by the inferior mesenteric artery, so they settle lower in the abdomen and take on a U-shaped configuration that spans the midline. Most people with this condition have no symptoms, and it’s the most frequently seen renal fusion abnormality on imaging. Other choices involve a fusion that’s less common (crossed fused ectopia) or conditions that are not fusion-related at all—duplex kidney is a duplicated collecting system within a kidney, and renal agenesis is the complete absence of a kidney.