Final answer:
tPA must be given for stroke within 3 or 4 hours to dissolve blood clots and restore blood flow, potentially preventing permanent brain damage.
Step-by-step explanation:
Tissue plasminogen activator (tPA) must be given for stroke within 3 or 4 hours of the stroke. If tPA is administered within this time frame, it can help dissolve a blood clot and restore blood flow to the brain, potentially preventing permanent brain damage. However, it is important to note that not all strokes are caused by blood clots, so the cause must be determined before treatment begins.