I'm going to dissent from the majority here, but let me preface what I say with this: I come from a family of nurses. Grandmother, aunts, uncles, cousins all in the profession. I know how skilled it is, and what hard work it can be. So in what I'm about to say, I mean no disrespect at all to my family, or anyone else who is a nurse.
I think you're being unreasonable to be disappointed, but I don't think you're being unreasonably to be fearful that she might not be fulfilling her entire potential, to capacity, with that career choice. I've worked in the NHS (not medical), with doctors and nurses, and there is a chasm between the best world-leading doctors and nurses - because there's a chasm between the best doctors and the vast majority of human beings! These are an elite bunch, they really are. (That doesn't mean that they are necessarily a nice bunch, mind).
I do think it's worth sitting your DD down and asking why she doesn't want to apply for medicine. It may be that the grades it requires are intimidating to her (despite the fact that it sounds like she could easily get them!) - someone's confidence level and their ability are two different things. If this is the problem, then would suggesting something like a gap year help to reduce that pressure? It might be something else - the hours, the state of the NHS - but she'll be exposed to that in nursing too. I think this needs further investigation, basically, to determine why she's making this decision and whether her reasons are rational.
However, if she can provide reasoned arguments why she wants to be a nurse, and they are based on fact not some misconception she has, then at the end of the day, you have to let her choose for herself.