Etiquette for returning to do research as grad student in same field as undergrad lab in same institution
Absolutely not rude or inappropriate, I'm a bit puzzled where you got the suggestion that it might be. There is also nothing wrong with going to a different group. You can certainly let your previous advisor know, and they would be a good reference for you who can speak about your research ability to other professors you are interested in working with, but you don't have any obligation to that professor to only ever work for them.

However, is it normal for people in medical school in your area to take a year off to do research? That part seems a bit more of a potential issue to me. Certainly there are joint research/medical programs (e.g. an MD/PhD in the US), but they typically involve much more than 1 year of research. If this is not a problem where you are, though, then there is nothing wrong with going back to a lab you have positive experience with.

One other thing to consider is whether you could gain some additional breadth of experience in another lab, but this is more about your future goals rather than an issue of what is "appropriate."