"Careful Doc, sometimes they shoot out of there at top speed..."
Dr. Chameleon's office is a dirty, cockroach infested bathroom in an old motel. He refuses his services to most mammals as they will likely eat him. A marmoset ate his receptionist after all, and that scenario replays in his mind when he's trying to fall asleep at night. As such, he is unrested and probably should not be examining cloacas at all, really. Combined with the fact that he never attended medical school of course.