how about this one then?

mentally ill man posing as doctor

It’s quite believable if you have worked in an Emergency Department. It’s just general chaos and no one knows who anyone else is, least of all the staff. It would be as easy as pie to just come in with a stethoscope around your neck, pull out the patient files and start seeing patients. All you’d need is a little experience of how the ED runs, which if you are a mentallly ill person, chances are you might have spent quite a bit of time waiting around Ed’s to learn how they work!