What is the best internship to get to prepare myself for an MD/MBA?

I'm a junior at an elite university, majoring in Economics, and I'm also premed. So far, I've done plenty of things on the medical side, and besides classwork, next to nothing on the economics side. I'm educating myself in Investment Banking and other finance fields over winter break to prepare for internship interviews in those fields. In general, I am applying for summer internships in finance (e.g. investment banking, asset management, insurance), consulting (healthcare or other), hospital (what types of internships do hospitals offer undergrads?), and pharmaceuticals/biotech/medtech. I plan on applying to medical school this summer, hopefully for a dual MD/MBA program. What type of internship, among those I just mentioned, would best prepare me for this path, and how can I make myself a competitive applicant for it? Thanks ahead of time for all your input!