If I did this again, I would impose an NDA so you won't get the prompt posted on the internet. (My prompts could not be prepared for in advance because ... I am a professional at this interview game.
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I am also mean enough to suggest 100 prompts, none of which you would get. For example, I can pull a pithy or inspirational quote and ask you to give your impressions... hundreds of possibilities. I could ask you about a topic I am covering on Becoming a Student Doctor of relevance to dentistry. I could pick a painting and ask you about it.
Approach it like your secondary essays if you were asked any from the school. If you didn't get any, feel free to stress yourself into a mess of goo looking up secondary prompt categories asked among medical schools.
We want to see if you can actually think on your feet and write clearly. Smart committee members might even be able to compare your writing against your personal essay where you benefited from help.
It is on the school to establish the parameters of the exercise. Don't stress out about it (more than you have to).
