That's the raffe line, it's the seam where the penis closed during gestation. Diaper rash can make it more noticeable.
It's a normal part of human anatomy, so there is no need to bring it up with the ped. However, if you want to see if your ped has a basic understanding of intact anatomy, go ahead and see if s/he knows what it is.
ETA: Gillian happens to type faster, spell better, and use more technical terms than I do (that's b.c she's a nurse,) so I deffer to her post.