Don't. Seriously don't. Read my previous posts about this matter. Unless you decline all offers because of a family/medical emergency you will burn all bridges at that school. Forever, probably. You risk getting blacklisted from all future activities there (med school, residency, fellowship, employment, etc.).
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success16, if you plan on a career in medicine, then the best choice is probably the biomedical research option. However, it depends on your career goals, how set you are in your path, etc. Research is research and will benefit you greatly in terms of the ability to problem solve. But, a recommendation for medical school will be likely perceived better coming from biomedical PI than an environmental one, and a biomedical experience will help you build a network in that field and this may help you later in life.
However, if you are not set in your decision to pursue medicine (if the environmental program seems more interesting than the biomedical sciences one, then this may be the case?), it will likely be beneficial to explore your options.
It also depends on which part of UG you are in. If you were a freshman, it probably wouldn't matter and I would tell you to go where you think you will enjoy your summer the most. As a sophomore or junior taking a year off (med school in two years, right?), you'd be better off, in terms of benefit to your application, probably going the AK route. (Though if you need a GPA-boosting science summer course badly enough, it might be worth it to go to MA.)
One more thing to consider, are you interested in epidemiology/environmental toxicology? Would the MA opportunity be related to this? If you can relate the research opportunity back to medicine/what you're going to do with your MD in 150 character, then this could be a viable option. Do you see yourself getting into (numbers-wise and during the year research experience/volunteering/clinical experience) a top 5 school where the MA experience could make your app more interesting and help it stand out?
Bottom line, choose the experience that seems most interesting. If this isn't the AK option, think about why that is (how strongly do you want to do medicine?). If you're not wholeheartedly interested in either, chose the one that will benefit you most in the future (as briefly outlined).