Wealthy individuals and celebrities might confront a rogue doctor whose best interest is not necessarily the wealthy individual's best interest. How does e.g. Donald Trump know that he can really trust his doctors? How does Trump know that one doctor has not secretly added lethal ingredients...

Just came back from the vet, I know they're not a traditional doctor but the same rule applies. These people, and I'm generalizing here, will do anything to make a few extra bucks off you. "Oh looks like you need this vaccine, oh it looks like there's an issue here too", "hey come back next...