In the fifth episode of Doctor Who Season 11 — "The Tsuranga Conundrum" — the 13th Doctor (Jodie Whittaker) wields the most doctor-ish accessory in the Time Lord's arsenal. No, not her sonic screwdriver or psychic paper. For the first time since 2011, the Doctor used a stethoscope as both a medical instrument and a sci-fi listening device.
Contemporary Doctor Who fans will remember David Tennant's popular 10th Doctor using a stethoscope a lot, most notably in "The Planet of the Ood" and "Midnight." But in the canon of Doctor Who, the Doctor using a stethoscope has a much longer history. In fact, the Doctor probably picked up this stethoscope in the 19th century from the creator of Sherlock Holmes, Sir Arthur Conan Doyle. But, here's the rub, that idea wasn't canon until 2013, even though the Doctor has been rocking a stethoscope since 1968.
On some level, a science fiction character named "the Doctor" wielding the most obvious medical tool of them all is kind of a visual joke. And the gag has been around for a long time.
The first appearance the Doctor's stethoscope was during the tenure of the 2nd Doctor, Patrick Troughton, in a 1968 serial called "Fury From the Deep." Weirdly or not, this was also the very first time the Doctor used the famous sonic screwdriver. "The Tsuranga Conundrum" seems to reference this, since the Doctor's sonic is temporarily put out of commission in this episode, meaning the Doctor can't use its scanning powers and instead relies on the stethoscope to listen to the spaceship's hull, and to check the heartbeat of General Eve Cicero (Suzanne Packer).
In "Fury From the Deep" the 2nd Doctor used the stethoscope to listen for weed creatures living in pipes. In the 4th Doctor (Tom Baker) adventure "The Creature From the Pit," the Doctor uses a stethoscope to listen to the hull of a ship, kind of like the 13th Doctor did in the new episode. The 11th Doctor (Matt Smith) briefly used a stethoscope in "The Lodger," but that was only to listen to the TARDIS itself. So, where did this stethoscope come from, and has the Doctor been using the same stethoscope this whole time?
The only origin story for the Doctor's stethoscope does not come from a TV episode, weirdly, but instead from an audiobook adventure featuring the 8th Doctor, as played by Paul McGann. Though McGann's only visual appearances as the 8th Doctor were in the 1996 TV movie and the 2013 minisode "The Night of the Doctor," he oddly has the distinction of having been the incumbent Doctor for the longest of any of the actors, because there is a nine-year gap between the TV movie and the first episode of the new series in 2005. What filled that gap were a bunch of novels and full-blown audio dramas from Big Finish, starring the 8th Doctor in the title role.
In 2001, an audio story called "Storm Warning," found the 8th Doctor revealing he'd "borrowed" the stethoscope from Sherlock Holmes author Arthur Conan Doyle and really meant to give it back at some point. This kind of thing happens a lot in Doctor Who -- the 10th Doctor said he got his big brown coat from Janis Joplin, whereas the 13th Doctor mentioned she gave Elvis Presley a cellphone in the recent episode "Rosa."
The point is, in Doctor Who canon, certain objects being connected to very famous historical figures is very common. So, if a 2001 audiobook says the Doctor got the stethoscope from the man who created Sherlock Holmes (Arthur Conan Doyle was a real-life physician, by the way), then does that mean Jodie Whittaker's 13th Doctor used that same Conan Doyle stethoscope in "The Tsuranga Conundrum," and that she has been since way back when she looked like Patrick Troughton?
Prior to 2013, the answer to this question would have been no. And that's because the Big Finish audiobooks were generally not considered part of the real canon of Doctor Who, which would have rendered the appearance of the stethoscope in "The Tsuranga Conundrum" a simple easter egg for the 2nd, 4th, 10th, and 11th Doctors.
However, in 2013, right before the big 50th-anniversary episode "The Day of the Doctor," the BBC released a surprise prologue, starring Paul McGann as the 8th Doctor. Though the "Night of the Doctor" was mostly designed to explain how the 8th Doctor became the War Doctor, it also effectively canonized all the Big Finish audio adventures.
In the moments just before the 8th Doctor regenerates in the War Doctor, he drops some names. Specifically, McGann's 8th Doctor apologizes to all his companions from the audiobooks, including a woman named "Charley," who he met in "Storm Warning," which is where the origin story for the stethoscope happened.
So, because Charley was mentioned in "The Night of the Doctor," everything that happened with the Doctor and Charley in the audiobooks is legit canon, including the detail he told her about borrowing his stethoscope from Sir Arthur Conan Doyle. Which means if Jodie Whittaker busts out the stethoscope in future episodes, we can all warmly think about her meaning to give it back to the guy who created Sherlock Holmes.
Who knows, maybe the Doctor accidentally encouraged Arthur Conan Doyle to quit being a medical doctor and take up writing. After some weirdo stole his stethoscope, maybe Doyle was sick of waiting for the TARDIS to reappear and decided to invent some blockbuster mysteries instead.