When Dr. Cox called the cast "the 4 horsewomen of the apocalypse" all of the colors of the horses are represented by what they are wearing.
Reid (white)
Carla (red)
JD (black)
Turk (green)
Mr. Foster, the patient getting an MRI, is played by Peter Jacobson. He also plays Dr. Chris Taub in the show, "House M.D."
This episode contains a scene with Zach Braff's character pushing Jason Bateman's character in a wheelchair. The same year this episode aired, Braff and Bateman costarred in the film The Ex, where Bateman plays a man in a wheelchair and Braff plays a character trying to prove that he's faking his handicap.
J.D.'s Social Security number is 987-65-4320 which are the phone numbers used in the song by the Clash.
The two patients spoken of at length in this episode are named Mr. Sutton and Mr. Foster. Incidentally, there is a prolific Broadway actress named Sutton Foster.