Can I reschedule a job interview?

This job I'm applying to has three rounds of interviews. The last is on the 30th and I said I would be available then. But I have a colonoscopy that I 1000% cannot reschedule, can't budge. I'm qualified for this job and would be making almost double my previous salary. But I'm afraid if I say "I have a medical situation and I'm not available on that day" might disqualify me. And I know we could say that if they're not considerate they're probably not worth working for, but I haven't had a full time job in more than a year and I'm trying to switch fields and this is one of my best shots. If I shouldn't say it's for medical reasons, what should I say?

usa jobs
usa jobs resume
usa hotel jobs
usajobs
usa jobs federal government
usa job in ksa
usa jobs
usa jobs login
usa jobs gov
usajobs.gov
www.usajobs.gov
usajobs.com
usajobs
usajobs.gov official