What is a stroke primarily caused by?

A stroke primarily occurs due to a lack of blood flow or oxygen to the brain, which is often caused by a blockage in the blood vessels (ischemic stroke) or a rupture of a blood vessel (hemorrhagic stroke). When the brain does not receive sufficient oxygen and nutrients, brain cells begin to die, leading to the symptoms associated with a stroke.

In the context of the other choices, excessive brain activity could lead to seizures but is not a direct cause of stroke. An infection in the brain, while it can cause significant complications and potentially contribute to strokes, is not one of the primary causes. Genetic predisposition can indeed play a role in increasing the risk of stroke, but it is not a direct cause; rather, it interacts with lifestyle factors and other health conditions that influence stroke risk. Thus, the primary mechanism behind a stroke is the interruption of blood flow, making the first choice the most accurate answer.