A compression of ILIAC VEIN that results in a decreased flow in the vein and in the left LOWER EXTREMITY due to a vascular malformation. It may result in left leg EDEMA, pain, iliofemoral DEEP VENOUS THROMBOSIS and POSTTHROMBOTIC SYNDROME. Compression of the left common ILIAC VEIN by the right common ILIAC ARTERY against the underlying fifth LUMBAR VERTEBRA is the typical underlying malformation.