I agree with some of you that a doctor do not need that you wear a brief to see what you have beneath your underwear, it is so easy to make you pull down your underwear if required.
if you are in brief, it gives to the nurse if they are performing the first part of the examination with you still wearing an underwear a better preview of your genitals, especially when you are standing on the scale or under the tape measure on the wall, but it is more easy for the doctor to instruct you to take off your underwear either a brief or an other one, so that your genitals would have no more secret for him.
Anyway, if I can now choose my kind of underwear before going to a doctor and wear a boxer or a boxer-brief if I prefer, when I was a child and a teenager in France, I had no choice, all boys except may be a few exceptions at that time like it is related by "Wrestlingpoet" were wearing white briefs type tighty whitey ones.
So we were rather exposed when undressed in underwears at school physicals, in lockers rooms, and so on each time we had to undress in group but we did not expect other situations at that time and did not think that it was possible to wear an other underwear more protecting than a white brief,
we were used to wear them, no matter if the doctor was not the only guy who could scrutinize all our bodies since we were also in briefs in front of the Pe teachers, the nurses and of course the other students.
and it is true that an embarrassing bulge could be sometimes visible in front of the brief but we might get over with that risk and I doubt that the doctor was surprised and interested (anyway there was a very few of boys who were circumcised in France)