With a tell-all book raising concerns about US President Trump's mental stability, there is a renewed question asking why the most powerful man in the world is not required to pass a thorough mental health exam.

Trump defended himself as a "very stable genius" in a tweetstorm over the weekend. But his remarks have done little to quell questions swirling about why the man with the nation's nuclear codes doesn't have to undergo more rigorous mental health evaluations.

"I think it's totally legitimate to ask about that and to have that as part of the examination," said Julian Zelizer, a professor of history and public affairs at Princeton University and a CNN contributor. "I generally think that once someone's president -- or once someone's even running for office -- this should be fair game. 

"This isn't about scandal, and it's not some kind of exposé. It's understanding whether someone is fit to hold the office and whether there is any problem that the president himself -- or his advisers, or the country -- should be aware of."