An asymptomatic 40-year-old woman undergoes a CT abdomen for unrelated reasons. Both kidneys are fused across the midline in the lower lumbar region, lying below the origin of the inferior mesenteric artery. The arrest of ascent occurs because:

• A The renal arteries fail to elongate, tethering the kidneys caudally
• B The metanephric blastema failed to induce ureteric bud branching
• C The kidneys are blocked by the duodenum during their ascent
• D The fused isthmus is hooked under the inferior mesenteric artery ✓

Explanation

In horseshoe kidney the lower poles fuse during ascent, usually anterior to the aorta. The resulting isthmus becomes caught against the root of the inferior mesenteric artery as it ascends from the pelvis, so the kidney comes to rest at the lower lumbar level rather than its normal position. Ureteric bud induction failure causes unilateral renal agenesis, not fusion. Patients have normal function but increased risk of hydronephrosis, stone formation and Wilms tumour.

Reference: Langman's Medical Embryology, 14th ed.

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Written and medically reviewed by the StethoPrep medical team.