3x4 What I Am
Addison and Derek are both horrified to see Mark shaking hands with the Chief, who's just hired him.
They're distracted by Meredith, who keeps throwing up after complaining of a stomachache. She collapses and Cristina asks her if she's pregnant, in earshot of everyone. Meredith says she can't be pregnant, but if she is, it could only be Derek's. She thinks she has an ulcer from trying to juggle Derek and Finn; it turns out she has appendicitis.