What best describes an ischemic stroke?

An ischemic stroke is characterized by the obstruction of blood flow to the brain due to a blood clot. This blockage prevents necessary oxygen and nutrients from reaching brain cells, which can lead to damage or death of those cells. The clot may form within an artery supplying blood to the brain (thrombotic stroke) or may travel from another part of the body, such as the heart, to block the artery (embolic stroke).

The distinction between ischemic strokes and other types, such as hemorrhagic strokes, where a blood vessel bursts (option A), is critical in understanding the underlying mechanisms of strokes. Furthermore, options like the formation of an aneurysm (option C) refers to a weakness in a blood vessel wall that can lead to hemorrhagic strokes if it ruptures, while a sudden drop in blood pressure (option D) does not directly cause a stroke but could contribute to other health complications. Hence, identifying that a blood clot obstructing a vessel is the defining feature of ischemic strokes clarifies the nature of this medical emergency.