The end credits for this episode play across a scene where Doctor Watson is deciding what title to give to the latest case.
Watson: And of course, catalepsy is a very easy complaint to imitate.
Sherlock: Yes, I know. I've done it myself.
This episode is based on Sir Arthur Conan Doyle's story The Resident Patient, which first appeared in The Strand magazine in August, 1893.
Click here to read the original story.