In episode 15 of the first season of House, Dr. House is served with a federal court order which specifically orders him to treat a particular patient who is needed as a witness in some kind of prosecution. (The patient is in a coma, and will not be able to testify unless he recovers.)

House is one of many doctors in the hospital, and there is no indication that he has any prior relationship with the patient. Doesn't this violate the Thirteenth Amendment?