I usually do not wear underwear and don't usually wear any for the doctor appointments, because these days I'm almost never asked to remove any clothing. Nor do they ever touch me or approach me during my exams. It all seems to be a mental health related thing. As in "how do you feel about your medications?" Or "do you think your condition can be attributed to....?"
Anyway I do have a few stories from years ago.
I recall several different nurses/assistants telling me to get undressed, BUT LEAVE YOUR UNDERWEAR ON, and put on the gown. They always just think everyone wears underwear?
I went in for an elective surgery on my upper back. I understood it would take less than 30 minutes but they would put me under anyway. I wore underwear that day. The very young nurse brought me into a curtain room, where everyone on the bay could hear everyone. She went over my vitals, meds, diet, prep, and talked about after surgery release and care. She then told me to remove everything, while pating her own hips, and telling me to only wear the gown and the surgical socks.
I didn't recall them asking me to remove the wrap around gown before they put me under. And I'm pretty sure they had to remove most of the gown to perform the operation. So I'm sure I was nude at some point in the operating room, while not awake?