My friends who went into the MBA program with just science backgrounds struggles the most in accounting (financial and managerial) and finance. More often accounting. I have to say, seeing the material for a 2nd time in grad school made it easier for me to digest (I had a business minor in college, economics major). So I'm going to suggest those 3 classes. Although these classes might have some prereqs in certain institutions.
But if you are involved in a program that offers all of the core classes this shouldn't be a problem. I find it strange that an MBA program wouldn't have any prereqs for someone to apply. Most MBA programs I have seen require experience and sometimes certain coursework.