I agree that your answer can deal with any aspect of your life and not just nursing. Certainly you must have had some difficult choices to make in your life. You life hasn't all be easy, has it?
For example, did you have a tough time deciding where to go to school ... or what to major in ... or whether or not to "follow the crowd" and do rebellious things as a teenage (like drink, smoke, cheat on a test, etc.)? You don't want your answer to reveal things about yourself that are best kept private, so pick something that you are willing to talk about publically -- and a time when the decision you made turned out to be a good one.
To answer the question ... recount the basic situation, the difficult aspect of the choice you had to make, how and why you decided as you did -- and how it all ended with you making a good choice and became stronger in the process.