This is maybe a naughty one but here you go:

2 men are waiting for doctor's appointment and they seem to have the same problem. They have a red ring on their p***s. Doctor comes in and goes with the first guy.
When he comes back he says:

"Oh what a relief, he said it was nothing."

Second guy goes in and doctor does his thing, after he was finished inspecting the p***s he says he has some kind of disorder in his p***s. The first guy is worried and asks why didn't the first guy have any disorder or any problems.

The doctor replies:

"I have been doctor for over 10 years and I can see the difference between lipstick and disorder."