GREY'S ANATOMY - "Oh, The Guilt" - Bailey treats a young mother battling breast cancer, Derek learns more about Mark and Addison's relationship, the interns defend a resident's competence at a hospital staff meeting, and Meredith and Addison examine a couple who find themselves in a rather awkward position, on "Grey's Anatomy," THURSDAY, OCTOBER 19 (9:00-10:01 p.m., ET) on the ABC Television Network. (ABC/RICHARD CARTWRIGHT) ELLEN POMEO, KATE WALSH