Usually if you have space you can add choices before the deadline or change them pretty easily. I did it a while ago for my first degree, it was so simple...just calling them and they were like yeah no problem. It was a quick alteration on their part.
But I didn't do it on the day it was due and it wasn't a course with an early deadline like med so I don't know how these two factors can change things but it's worth a try.
Call them today, the deadline is 6pm I'm sure they can do it. Make sure you call them early, I can assume deadline day is probably hectic.