Through this question, the AD Com want to know about your personality and your non-work life. Since the school asks you to discuss moments that have defined your personality, you may share more than one life -experiences that have shaped your personality. You feel so strongly about your story of selection of under- graduation courses, so go ahead and describe it briefly. In addition, I would encourage you to do some more introspection and identify other moments/ incidents that have had a major influence on your personality. Explain how you dealt with that particular situation ,and how it taught you meaningful life lessons that you are using effectively in your personal and professional life.
I hope it helped.
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