"Doctor" is among the thousands of job titles Moos appoint to themselves which they magically acquire the moment they sluice and not through actual experience or education. Doesn't surprise me in the least when Moos march into doctors' offices, tell the providers what they've diagnosed their child with via Google and then proceed to argue about it when the doctor doesn't agree with them, frequently citing their Moo status to refute logical medical claims.
Contrary to popular breeder belief, mother does not know best. In fact, unless they happen to professionally specialize in an area of expertise, she probably doesn't know at all. But ignorance never stopped proud Moos from acting like they know everything just because they bred.
Well if Mommy is so damn sure what's wrong with her loaf, then why bother taking him to a doctor at all? And of course she doesn't want to get testing done to find out more because then she might be proven wrong! It has nothing to do with how hard it would be for the kid - she just doesn't want concrete evidence of her stupidity presented to her. So instead of getting the kid a real, helpful diagnosis, she'll just neglect his needs because no doctor will tell her what she wants to hear. I also imagine she took offense to the possibility of a genetic defect because it would possibly mean she was at fault for her kid's ailment (even if she didn't try to be).
I think it's okay to bring up concerns to a doctor, but only to discuss them as possibilities, not to tell the doctor how to do their jobs. I don't think it could hurt to say, "I looked up his symptoms and found this, this, and this. Should I be concerned about any of those?" Meet them halfway, not, "MOOOOO I ARE MOMMY, HE HAS THIS AND YOU'RE WRONG IF YOU SAY OTHERWISE!!"