To the people who second guess the doctors, what if the doctor was a proper hyperbaric doctor?
Again, there are a lot of different scenarios here. It is my belief/hope that most responsible instructors that find the need to refuse a student for training due to medical concerns even with MD sign-off, are doing so as part of a discussion of those concerns that involves the diver, diver's parents if a minor, and where applicable the MD and/or DAN.
If someone is making an apparently random refusal due to a medical concern apparently on a whim, I'll agree that there might be a bit of ego involved and probably not someone I want in my operation, even if they do have that option. (I do know a pretty good instructor that just refuses to train any minor divers under a certain age. We have not discussed it at length for me to cite exactly what age, or the reasons involved.)
Success in this (and any) business relies on trying to find ways to "yes" to the customer. So off-hand dismissal of a potential customer is poor business. But carefully considered, reasoned, and communicated refusal if necessary is the responsible thing to do.
So, a "proper hyperbaric doctor" would go a LONG way towards resolving any hesitancy I might have IF the issue came up. I have never turned away anyone for a medical reason, but I do reserve the option to do so if I just don't feel right about bringing a student underwater no matter what anyone else says. I just doubt that will happen.