When I went through it a few years ago, if you accepted an internship, the Case Study course was mandatory. The fall class was not mandatory, but you could not get the Internal Audit concentration if you did not complete it.
This is pretty much how it goes I think.
If you accepted an internship, you could probably get away with not taking the Case Studies class, but Sumners would probably be pretty mad about it and might alert your hiring company. Depending on their opinion of the value of the CIA program, they might be mad that you wouldn't be completing the program.
As for the getting the concentration, yes, Sumners would definitely require you to take the case studies course to finish the program and "get the concentration." However, I don't think the "concentration" is officially recognized by the university. To my knowledge, there is nothing marked on my degree, transcript or anything else that denotes the fact that I completed the entire CIA program. I still put it down on my resume and my bio for work/clients, but I dont think there is an official academic program defined by the Registrar's office -- just a designation made up by the CIA program itself and endorsed by the Business College.
Does that make any sense?