May–Thurner syndrome (MTS), also known as the iliac vein compression syndrome, is a condition in which compression of the common venous outflow tract of the left lower extremity may cause discomfort, swelling, pain or clots (deep venous thrombosis) in the iliofemoral veins. Specifically, the problem is due to left common iliac vein compression by the overlying right common iliac artery. [Source: Wikipedia ]