Patient: Doctor, every time I look in a mirror, I feel ill, as if I'm about to throw up. What's wrong with me?
Doctor: I don't know, but your eyesight is perfect.
Mirror, Mirror
Explanation
Explain Bear
Listen up, buttercup! The patient thinks he's sick because of what he sees in the mirror, but the doc is basically saying he's ugly. It's a burn disguised as a checkup, you walnut!
Neighborhood asshole
Good on the doctor for complementing them on their eyesight