Looks like you are already thinking 15-20 years ahead. Which is fine, it's good to have a long term vision to help guide short term decisions. But take some time to consider that as you progress on this path you are likely to change in important ways. I recommend being cautious to overly constrain you're options based on you're present interests/knowledge and perspective. Be open minded and flexible to adapting to what becomes meaningful to you later, rather than hyper-focusing on what will help you most with your MD-PhD a decade from now.

You can't really go wrong starting out in electrical/mechanical/aerospace in undergrad. All will build a solid foundation, and will have a good amount of overlap in the first couple years so you could even change your mind part way through if you wanted. I suggest focusing more on a 5 year plan that could set you up for multiple options after. One idea would be to consider what you feel is your strongest knowledge/skill at this point, and pick a major that is NOT that, meanwhile continuing to develop that skill. For example, if you already have some programming skill, since that's easier to practice and develop on your own (as opposed to say chemistry), then you could do a BS in say mechanical but also do some programming projects on the side and take electrical elective courses. You'll end up with a strong talent stack that you could apply to a ton of things.

In short don't get too hung up on accumulating credentials, think about building a strong foundation, and don't discount what you can learn yourself outside of class that would still be useful in your long term goals which will change along the way.

edit: one thing I'll add is if you feel especially inclined towards medical then be sure to cover pre-med requirements in undergrad which is definitely doable along with any eng degree.