I've been wondering how often there are examples of isst vs. ist being misunderstood in speech. I listened and thought "The doctor is no fruit" was weird, but maybe not too weird for Duolingo. Maybe some kind of "The doctor is a fruit" is some kind of insult. What do I know. The "correct" answer sure made more sense. Interestingly, Duolingo did not mark "is" wrong in this case.
Medic is Mediziner(in) or Sanitäter(in) (the latter mainly referring to paramedics), according to dict.cc.
Anyway, if you think something should be accepted when it isn't, it only has a chance of changing if you hit "report a problem" - mentioning the issue here isn't going to do anything.