What business can a nurse start?
First, you can start any business you want, regardless of the limiting idea that it has to be related to your nursing background.
Second, if you are dead set, tunnel vision, wanting to leverage your knowledge as a nurse to start a business, then I would guess the first question would be to find out what type of nurse you are and what skill set you have.
Based, on the response, to #2 above, I would then present a myriad of ideas. But you have the background, knowledge and credentials to start countless businesses.
You don’t need the nursing skills, unless you plan on doing the work yourself. You need the business building skills to build any type of business you want.