GREY'S ANATOMY - "Beat Your Heart Out" - Derek nearly catches Mark and Lexie flirting. Previously, Derek had forbidden Mark from having a relationship (or whatever passes for one with Mark), with "little Grey," on "Grey's Anatomy," THURSDAY, FEBRUARY 5 (9:00-10:02 p.m., ET) on the ABC Television Network. (ABC/RANDY HOLMES) ERIC DANE, PATRICK DEMPSEY, CHYLER LEIGH