Doctor.

Jonathan goes to the doctors and says, "Doctor, I've got this problem you see, only you've got to promise not to laugh."
The doctor replies, "Of course I won't laugh, that would be thoroughly unprofessional. In over twenty years of being a doctor I've never laughed at a patient."
"OK then" says Jonathan, and he drops his trousers.
The doctor is greeted by the sight of the tiniest penis he has ever seen in his life. Unable to control himself, he falls about laughing on the floor.
Ten minutes later he is able to struggle up to his feet and wipe the tears from his eyes, "I'm so sorry," he says to Jonathan, "I don't know what came over me, I won't let it happen again. Now what seems to be the problem?"
Jonathan looks up at the doctor sadly and says, "It's swollen."