#1: Harassment
#2: Customer service complaint
#3: Dishonest coworker
#4: Tutoring
My thoughts:
I like #1 and #4 the most. #1 sounds genuine, though needs to be written tactfully. #4 is more run of the mill. Both can demonstrate personal characteristics that are highly desired in physicians. Depending on what the complaint was and how you bounced back, #2 may potentially also work.