GREY'S ANATOMY - "Tainted Obligation" - When Meredith and Lexie's father, Thatcher, returns to the hospital with a failed liver, it's up to Meredith to save his life. Meanwhile, Izzie empathizes with a patient riddled with tumors, as Mark, annoyed with Cristina's competitive zeal, tricks her into assisting on an unusual surgery, on "Grey's Anatomy," THURSDAY, OCTOBER 8 (9:00-10:01 p.m., ET) on the ABC Television Network. (ABC/MICHAEL DESMOND) CHYLER LEIGH, JEFF PERRY