Which one presumably determines by, y'know, testing for the deficiency. But that's never medical necessary, so they won't pay for it as part of preventative care.
But hey, we have the best health care system in the world! 'Murica!
(I am paying for this test out of pocket, incidentally, because the doctor ordered it, because I have consistently low vitamin D and am on supplements. But don't worry, they're clearly not medically necessary!)