azygos continuation (of the inferior vena cava)

az·y·gos con·tin·u·a·tion (of the in·fe·ri·or ve·na ca·va)

a congenital anomaly in which the infrahepatic portion of the vena cava fails to form, and venous drainage of the lower body is maintained through a persistent right supracardinal vein, which becomes a large azygos vein.