In May-Thurner syndrome, which vascular relationship predisposes to left-sided DVT?

• A. Left common iliac vein compression by the right common iliac artery
• B. Left common iliac artery compression by the right common iliac vein
• C. Right common iliac vein compression by the left common iliac artery
• D. Inferior vena cava compression by the aorta

Answer: (A)

May-Thurner syndrome happens when the right common iliac artery crosses over and chronically compresses the left common iliac vein against the lumbar spine. This extrinsic compression narrows the left iliac vein, causing venous stasis and endothelial irritation, which promotes thrombus formation and predisposes to left-sided deep venous thrombosis. Over time, the ongoing pulsatile compression can lead to intimal hyperplasia and intraluminal webs that perpetuate obstruction. The other vascular relationships don’t produce this left-sided venous outflow problem—the key is the right artery compressing the left vein at the pelvis, not compression in the opposite direction or involving the IVC.