If people are surviving their kidney, heart, lung problems, they live longer and potentially develop brain problems. Some people that would otherwise be dead will now be living with an uncurable brain disorder, increasing the percentage of the population with brain disorders.
This assumes that brain disorders are not curable as well. If we could cure brain disorders just like we can cure kidney disorders, the proportion wouldn't rise. If we assume C), there will be people with uncurable brain disorders, because brain transplants are impossible.