I agree with llg. Just because you blew a scenario doesn't mean you blew the interview, they want to know how you think, and what YOU prioritize as most important. They also want to know what you learned in school.
Having said that, let me stress for me, Pain is a huge concern because infection or not, Patient Comfort is number one with ME. First thing I take care of is my patient, then I handle the problem. If his pain level is a 7-10 I need to manage that first because anything I do for the infection or anything else could drive his pain scale even higher. It all depends on the scenario.
For my school, Pain is a huge priority followed by infection, then everything else. Once the pain is managed, then the infection can be managed.
Being it was 100.9, and not 101 or higher, Pain would be my choice followed by inspection of the incision site, followed by inspection of the lab values (hence assessments) to see if the pain is being caused by a infection.