Definitely call up the medical school and explain to them your situation. If you explain it to them, they may give you the benefit of a doubt since you've already been accepted and decided to come clean about it. I just don't see them rescinding the offer if you can honestly prove to them that you didn't understand that going abroad would prevent you from matriculating at a US medical school.
But if you did do a year of med school abroad and then came to the US, and then applied for residency, with this whole thing hanging over your head, there are going to be too many questions that are going to be raised. You never know when you're going to run into a former classmate or if someone does an audit on your application and discover that you went abroad. If you get caught in a lie like that, that'll definitely be a huge problem getting your license or completing your residency. Come clean about it. In medicine, as in politics, you get into more trouble for covering up something than for the actual trangression itself.