A routine ultrasound in a young adult shows a single kidney mass lying in the lower lumbar region, with both ureters entering the bladder normally. CT confirms fusion of the two kidneys across the midline. The low position of this fused kidney is best explained by:

• A Excessive ascent beyond the diaphragm with secondary fusion
• B Failure of the ureteric buds to induce the metanephric blastema
• C Persistence of the cloaca preventing pelvic descent
• D Arrested ascent of the metanephric kidneys against the developing inferior mesenteric artery ✓

Explanation

The metanephric kidneys ascend from the pelvis to the upper lumbar region during weeks 6 to 9. In horseshoe kidney the lower poles fuse across the midline, and during ascent the isthmus catches on the inferior mesenteric artery, leaving the organ at the level of the lower lumbar vertebrae. Failure of ureteric bud induction causes agenesis or dysplasia, not fusion.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.