This is very questionable advice to give or consider in the long term. Withholding facts is the same as lying in the eyes of the military, because you sign that medical form saying you have been truthful and complete. Sure you might get away with them not finding out about something you withhold for the NROTC exam, but what about the many other exams you will go through in your military career? When it is discovered and you say "Oh I've had that for years no big deal" will they believe you? If they do, how well do you think that will go over that you have left out medical information? Mols.cast8, I see that you are going to USNA Foundation and just starting the adulting thing, so I understand you are learning. I would recommend you consider that the first word of the US Navy core values is Honor. Is withholding medical information and instructing a fellow future Midshipman to do the same follow that core value?
To the OP, Be smart..Be honest. If they do ask for additional medical information, just do what they ask. It is all you can do. It sounds like it is a minor issue if you have had it for most of your life and it doesn't affect you. Try not to stress about it, and you will be OK. What sounds like a benign cyst is alot different than a peanut allergy or replacement joint as far as getting a waiver.