I think you are confusing what is presented in BU, you have very little choice in what you are allowed to do. There's some space where you can pick one elective over another but all in all, you are spoon-fed what to do, who your advisor is and pretty much everything else. It is both more organized and at the same time if you want, can make it more flexible.
All students are required to take some form of histology, physiology and biochemistry. If you're taking the 1 semester histo/physio, you will take pharm as your default elective unless you personally choose to take something else. If you take the 2 semester histo/physio, you will take immunology as your default elective, once again, unless you choose to take something else.
You do not choose your classes. You are given a pre-decided schedule including what track you will be in and who your advisor will be and this also includes your electives. They are all decided for you and 98% of the students will follow this route. Some will decide to deviate but that is by THEIR choice and not a default choice by every student.
These options include replacing pharm/immuno with another elective, doing a concentration, or doing a MPH. However, very little students deviate from the set route handed from you. If anything, the only choice you should TRY to make is whether you want a 2 semester histo/phyhsio or a 1 semester histo/physio. However, even that is difficult to choose because of space constraint.
I'll make sure to reiterate this for both of you guys because for some reason people seem to think otherwise. Everything is predetermined for you and unless you CHOOSE to do something different, you will follow that course. You are handed your advisor, your track, your schedule, and basically what you'll be doing for a year ... about a MONTH before you even go to school. If you're like me or most of us, you won't deviate from that and most likely you won't even take more courses than that. BUT, if you want to do things differently, there is SOME room for that to be possible.