artery here's the blood vessels in the brain atherosclerosis can happen anywhere along this tree that's the number one cause of stroke atherosclerosis next most common is going to be an embolism from the heart an embolism means a clot that started in one spot and it travels somewhere else so the most common heart condition that could cause a clot to form in the heart is called atrial fibrillation so atrial fibrillation is when the top chambers of the heart instead of beating nice and regular like this start to do