The single best piece of advice (and most successful strategy) I’ve seen, is to write an explanatory addendum. Don’t make excuses, don’t lie, don’t try to blame others. Take blame for what you did, explain why you did it, and explain what you learned from the mistake and why it won’t happen again.
Anything else will probably not be received well. We’ve worked with students with these sorts of problems, and I’ve seen them get admitted to the very top medical schools regardless. Even one student who had been suspended for plagiarism/cheating (this is really not a good thing in graduate school or medical school admissions). In any case, honesty, maturity, and straightforwardness will benefit you greatly.
Hope that helps.
If you want help writing an explanatory addendum, we have a few counselors that could certainly help – I would work with one of the MD/PhD Admissions Team …they can probably handle it best.