Thats a tough question, I would go with something that made you decide to become a nurse. But, in a way that talks about how along the way life made you prepared to become a nurse. Like, how something happened that you learned from. Or, how you volunteered at such-n-such and it made you realize that you wanted to help people. Maybe even, a horrible event that made you "grow up".
My best suggestion is to write about something compelling on how you are prepared to take the next step and become someone greater than you are now. How your life journey brings you to apply for nursing. something like that.