It wants you to use ことだ here, but putting in べきだ should be right as well based on my understanding of that grammar point. It means that you “should” do something based on pretty much common sense. Therefore I feel like you should be able to say basically that it’s common sense to take the medicine because it was a doctor that prescribed it. I get in the explanation it says that it wants the appeal to authority one, but I thought べきだ should maybe be yellowed as an alternative instead of red and completely wrong
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Thanks for the feedback! Indeed there should be a leading-hint for this input (orange text hint).
I’ve passed this onto the Content team. 
In the future, you can submit feedback like this a lot easier by submitting a Report, like so:
This will make sure it goes straight to the right team, along with the sentence in question.
Have a great day, and happy studying.
Sean
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