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I think it might be a mistake to use only one acheivement to answer that question. The questions seems to be asking what your best acheivements are, it is not specifically asking for one personal and one professional. I interpret it as simply asking for you to explain the great things you have done. By highlighting only one, the adcom might see you as not deep enough with experiences.
500 words on a single event might also come off as overly verbose and you potentially expose yourself to prose that is slightly repetitive and played out. Remember that big achievements most often speak volumes for themselves, spend what few words you have on showing the adcom how rich your experiences have been. Spend the more words on the experience you have mentioned since it is the most important, but make sure the adcom sees the other experiences that have shaped you as well.