Plaques can cause several problems. First, plaques can protrude into the artery, eventually causing a partial or complete obstruction to blood flow. Second, plaques can suddenly rupture, causing a blood clot (thrombus) to form, leading to sudden occlusion of the artery. (This condition is called arterial thrombosis.) Third, plaques can weaken the wall of the artery causing a ballooning out of the artery to form what is called an aneurysm. The rupturing of an aneuyrsm often produces bleeding that requires emergency treatment.