Anatomical features of coexisting horseshoe kidney and double inferior vena cava.

To describe a rare case of coexisting horseshoe kidney (HSK) and double inferior vena cava (DIVC) observed during a cadaveric dissection course and to analyze the embryological and clinical implications of this anatomical variation. A detailed anatomical dissection of a 96-year-old Japanese male cadaver was performed during a gross anatomy course. The kidneys were fused by a fibrous and parenchymal isthmus below the inferior mesenteric artery. Four right renal arteries and three left renal arteries were identified, along with an additional renal vein arising from the isthmus and coursing to the left adrenal gland. The right and the left IVCs ascended bilaterally along the aorta, with no communicating veins observed. Histological findings revealed atrophic renal tubules in the isthmus, with increased connective tissue. This report highlights the high variability of renal and vascular anatomy in coexisting HSK and DIVC. Recognition of such variations is essential for radiologists and surgeons to avoid complications during renal transplantation, vascular surgery, and imaging interpretation. In addition, the kidneys and the IVC development occur simultaneously between weeks 4 and 8 of embryogenesis. Therefore, abnormalities of retroperitoneal structures that occur during embryological development can lead to variations.