I'm assuming that your experience as a nurse was under socialized medicine. Any of the areas you indicated would be good. I just would recommend you start out working in a hospital environment to get the full impact of the U.S. version of medicine. The type of hospital, i.e. government-supported, nonprofit, for profit, and so on, is what you want to pay closest attention to. This will affect how you can administer care in many cases or even the kinds of patients and families you serve. Best wishes.