It depends on whether you have other more value-added material that will get bumped by focusing the optional essay on your medical condition. However, since you say the condition was life-changing it sounds like it would be something you'd want to discuss at length in an essay. If you've already done so in one of the non-optional essays, then you may want to focus the optional essay on other, new material.
Hope this helps,
Co-Author of: MBA Admission for Smarties: The No-Nonsense Guide to Acceptance at Top Business Schools
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