Knock, knock. Who's there? Dave. Dave who? Dave proceeds to break into tears as his grandmother's Alzheimer's has progressed to the point where she can no longer remember him.
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Explanation
This is an anti-joke because it sets up the typical structure of a knock-knock joke, but instead of a funny punchline, it ends with a sad and poignant scenario where Dave's grandma doesn't remember him due to Alzheimer's.