Just remember that they've seen hundreds in their career, (probably a few a day for a GP, or at least a few a week!), and they'll even have seen and maybe even dissected a few dead ones before. It's really not a big deal, seriously as far as the doctor is concerned its no different to you showing him/her your foot.
I've had to show a doctor my penis a few times before (had a circumcision recently, so had a few appointments with various different doctors where I had to whip it out). It's not the most natural thing in the world, but just remember that the doctor is literally not bothered about seeing it, and you'll be fine. I can think of some other things that are a lot more embarrassing that you could have to show/do in front of a doctor.