Atherosclerosis causes plaque to form within the carotid artery walls, usually at the fork where the common carotid artery divides into the internal and external carotid artery. The plaque build up can narrow or constrict the artery lumen, a condition called stenosis. Rupture of the plaque can cause the formation of a blood clot in the artery. A piece of the formed blood clot often breaks off and travels (embolizes) up through the internal carotid artery into the brain, where it blocks circulation, and can cause death of the brain tissue, a condition referred to as ischemic stroke.