"Tainted Obligation" is the fourth episode of the sixth season of the American television medical drama Grey's Anatomy, and the show's 106th episode overall. It was written by Jenna Bans and directed by Tom Verica. The episode was originally broadcast on the American Broadcasting Company (ABC) in the United States on October 8, 2009. In the episode, Dr. Meredith Grey (Ellen Pompeo) and Dr. Lexie Grey (Chyler Leigh)'s father is admitted into the hospital, seeking a liver transplantation.