<prompt> In case of IVC obstruction the collaterals which open up are all except ?Superior epigastric inferior epigastric veinAzygos ascending lumbar veinSuperficial epigastric iliolumbar veinLateral thoracic veins preveebral vein  <response>: Superficial epigastric pain drains into great saphenous vein. Iliolumbar vein drains into internal iliac vein,so these two do not form collateral between SVC IVC as both of them will be ultimately drained into IVC.All other choices form collaterals between SVC IVC. 