If you don't think you'll be able to finish the PhD, then get out of it. Instead of just dropping out, though, you might want to consider transferring to a masters program. Even if there isn't a masters track in your specific graduate program it is usually possible to transfer to a different program entirely. Since you've already been admitted to the graduate school it is not that complicated a process (at least at my university).
If you want to get into something relevant to medical school, you might want to consider something like a masters degree in pathology or anatomy. I've known someone in a similar situation as you who did that and it seemed to work out well for him.
You can prepare for your MCAT and do your shadowing while you're doing your masters degree, I imagine, and it will look better than simply dropping out of school.