I was about 18 with friend's Sister.
Just out of school my best friend's sister was in med school at USC. I was 18 or 19 at the time. Back then I was pretty fit and buff from working as framing carpenter for the summer. Trish was her name and she either was a doctor or just becoming a doctor, maybe in residence or something like that.
Trish asked me if I'd be willing to submit to a physical for her. At first she asked me to come to her class but I didn't think I'd be comfortable being naked in front of so many people. From what I recall the pay would have been good but at that age I was still bashful. Much unlike today.
As the days went on the requests changed until she said it would just be her and me. I accepted as a volunteer and had only one condition, she wouldn't do a DRE. She said fine.
I met her at either the school or hospital, I was never sure what buildings were what. She took me into an exam room and asked me to undress. I was reading a magazine when in walked a young nurse to take my blood pressure and temp. I was a bit surprised but later learned that the doctor would also check her findings.
I started to get a semi-hard while standing on the scale and turned so the nurse wouldn't see my cock starting to make its way to full mast. It was about half erect and I think she saw it. Quickly, she said she was done and walked out. I went back to sitting on the cold paper my dick deflated.
Finally Trish walked in. She did all the things that the nurse did and again I was standing on the scale and said when the nurse did it I got a bit of a hard on. I asked her if that was normal.
Trish just said she'd seen her share of erections. They were taught not to make anything of it if it did happen so as not to embarrass the patient and sometimes they would walk out of the exam room with an excuse and come back in a minute or two. She said usually that worked but some guys stayed hard throughout. The younger ones especially.
Up until now Trish had never seen me naked except once while swimming in a mountain lake and once getting in and out of the hot tub. Both times were just quick glances so I already knew her carpet matched the drapes.
She told me she didn't remember me being so large before. Winking at my Penis that was poking out from under the gown. I looked down and saw it, not realizing it was starting to get hard again.
I laughed and said the last time I was with her I was cold.
She did all the usual doctor stuff and then palpated my penis and felt my testicles and then she said she really should check my prostate. I told her fine as long as a finger wasn't going anywhere near my ass. She said there was only one way to do it that she knew and it would require a finger.
At the time I was dead set against it because I just assumed it would hurt. I did relent after she promised I could say stop as soon as it hurt.
She had me lay on my side with my butt towards her and in went a gloved finger. I never felt any pain at all, in fact it started to feel good. I told her so and she quipped she might be able to make me shoot without coming. I said there wouldn't be much fun in that. So she said wait and see.
As she was massaging my prostate my cum just started to pee out of me, there was that much! My cock was now rock hard and she reached around with her left hand and grabbed it. She did some squeezing along with some stroking and then I shot off and semen actually hit the wall next to me.
I told her I didn't think that was the usual protocol. She laughed. We promised not to tell any one else about this, even her bro.
Here I am twenty years later and this is my first time telling the story. Trish isn't a doctor anymore, I'm not sure why. I haven't seen her once since a few years after that episode.
To this day when I walk into the doctor's office my cock tingles and wants to get hard.