The first time I remember having to get undressed to my briefs was when I was about 11 and my mom was worried about my growth. She took me to a pediatrician in a small city near our small town, and the nurse made me get undressed down to my white briefs right at the start.
Most of my other exams as a kid were done with me shirtless ,and the trousers came down only briefly.
The next time I recall having a physical in just my undies was when I was 24 and returning to grad school. I remember the doc being pretty curt, since I wasn't going to be a patient of his. I was nervous and turned on at the same time because the physical form had a "rectal" entry on it. I remember the doc filling out the form in front of me after my physical appeared to be done, pausing at the rectal line, and just checking it off as "normal." I was relieved and disappointed at the same time.
I think I had another physical in my briefs in my late 20s.
When I started going to the doctor in my mid-30s after my father got diagnosed with prostate cancer, my doctors started using a gown, for good or for bad. I was told to undress, "including your underwear," slip into the gown, opening in the back, and have a seat. If it was my first exam with that doc, I'd meet him while dressed first and he'd tell me to get undressed and put on the gown, and leave the room while I did so.
But there was one memorable exam (which I've posted about elsewhere) in which the doc came in while I was fully dressed, and then directed me to get undressed. I was expecting him to offer me a gown and then leave the room, but he stayed.
The doctor watched me very closely as I removed my shirt, shoes, socks, and jeans.
I was wearing black boxer briefs. I remember pausing and thinking about whether I should ask, "These too?" If this exam had happened today, I would've just taken them off and waited for the doctor to try to stop me, the thought being that I had been directed to remove my underwear in previous physicals, so why not this one.
But, I just took a seat on the end of the exam table in just my black boxer-briefs. I do remember the doctor giving me a pretty thorough genitalia exam with my boxers at my knees and me lying down. That was hot, because my previous such exams had been pretty cursory and with me standing.