It seems unlikely that your school wouldn't offer some sort of "replacement" class for a course that was mandatory for graduation, otherwise a bunch of people wouldn't graduate that year! I would check with your academic advisor or department chair (for your major) regarding what other class(es) would fulfill that requirement.
I think all of the acceptance letters I've seen stipulated that acceptance was contingent upon completing the degree, and if it wasn't completed before med school started, the school could recind their offer of a seat in the class. (Some schools also listed additional requirements, such as not receiving anything lower than a C in classes taken during the application year, etc.) I think it would be pretty unlikely that schools would allow you to defer because you didn't complete your degree (unless you could document unusual circumstances like a death in the family, etc.). Deferral policies vary from school to school, but usually not completing your degree is not a sufficient reason (you should have been able to find another substitute class, etc.). There have been lots of threads on deferral--a search should turn up those threads.
