Final answer:
Ischemic strokes constitute 90% of all stroke cases and happen due to a blood clot preventing blood flow to the brain, leading to irreversible tissue damage in the ischemic core.
Step-by-step explanation:
Ischemic strokes affect about 90% of individuals with a stroke. They are typically caused by a thrombosis or embolic occlusion of a cerebral blood vessel, leading to a restriction of blood flow to the brain and reduced delivery of oxygen and glucose. This lack of blood flow results in an ischemic core with irreversibly damaged tissue. However, surrounding this core is the ischemic penumbra, an area where tissue is impaired but potentially salvageable if treatment such as recombinant tissue plasminogen activator (rtPA) is administered promptly.