If they have got foot in mouth disease, they may very well not know, so you can "politely" tell them it isn't "polite" to assume because you see a scar on my tummy, that I have had a tummy tuck!
When we are not well, we are vulnerable, tired and weak and often we are "more" polite because we don't have the energy nor inclination to correct people the same way we may when we are not in a vulnerable position, I get that!
But if these comments are happening quite a bit to you @Dh5Wy83 be prepared with a response. It is OK to tell them what the scar is from, because after all they are doing the tests and want to know what he scar is a result of, unfortunately they have jumped to conclusions which is not professional at all.
All the tests you are having are exhausting, so also give yourself some grace, and try not to let their comments get to you. You have been through a lot and I hope you get to the bottom of your health concerns soon and get the treatment you need very soon.