Maybe a short paragraph. You could sort of make it the theme, like use it in the introduction and then explain all the things that happened afterwards that made you want to be a doctor and conclude with how even with 3 bad semesters you still decided you were going for it because of all the stuff you mentioned in the body of the essay. I think that could be good. Just make sure you mention a lot of other things. But, a lot of schools just send all applicants secondaries so you will be able to explain it thoroughly there.