Unless I'm confused, this definitely needs that knowledge
tag, because I think I am
a doctor
and this puzzle is about
the various times Dr McCoy said "I'm a doctor, not a ..." on Star Trek.
Consulting
the start of the list at https://www.ditl.org/paglistdoctornota.php (I have no idea how authoritative it is)
we see:
House builder: "I'm a doctor, not a bricklayer!" (The Devil in the Dark)
Freud: "I'm a surgeon, not a psychiatrist!" (The City on the Edge of Forever)
Einstein: "I'm not a scientist or a physicist, Mr. Spock..." (Metamorphosis)
Moving staircase: "I'm a doctor, not a escalator!" (Friday's Child)
Carrack: "I'm a doctor, not a mechanic!" (The Doomsday Machine)
Hoist with one's own petar (this is not a typo, it's how Shakespeare wrote it): "I'm a Doctor, not an engineer!" (Mirror, Mirror)
Copperfield: "I'm not a magician, Spock, just an old country doctor." (The Deadly Years)
Loretta Lynn's father: "I'm a doctor, not a coal miner!" (The Empath)
This leaves
Annie Sullivan (must be "miracle worker")
but
since OP, in comments on the question, seems keen to insist that there's "nothing particularly obscure" here, I shall refrain from going to further effort to try to determine where that one occurs :-).