What I’m trying to say, without using a sledgehammer, is that if the person who had been in nursing is able to achieve the extremely high grades in the very challenging premed prereqs, and a high MCAT score, the reason that the person wants to move out of med school and into nursing will be obvious to those making the decision.
But to state it bluntly, the person would likely explain in a delicate, polite, and careful manner that they are interested in the diagnostic challenge involved in being a medical doctor, the possibility of becoming a surgeon, or performing complex medical procedures, of leading the team caring for the patient, etc, etc, etc. The achievement that makes admission to medical school a possibility states in and of itself that the person has the intellectual capacity to become an MD, wanted the challenges, satisfaction, and rewards that come along with that path, and so switched.