The two biggies in our first semester were biochem and histology. Like I said in another thread, for me biochem was very challenging. I wish I had taken it in undergrad to at least be somewhat familiar with all those pathways you have to memorize. Histo wasn't too bad, if you were a bio major I think you have been exposed to a lot of that stuff before. But if not I can see it being a little tough as well. So I would say it depends on your background. Check with your school to see what classes your going to have and then plan accordingly. I would say any upper level science, anatomy, histo, micro might be a wise choice.