Dima88,
Once its on paper with one authority , you now have no choice but to declare your passed medical issues.
If you dont declare it, and down the track an investigation takes place, lets say after a crash (maybe not directly related to you) the insurance company for example will not pay out.....
Your medical records are on record (computer) they will know everything when you make an appointment .
My advise is to do what they said and get further medical advise about your kidneys