Story highlights "If he was unfit, he probably wouldn't be sitting there," Sanders said

CNN has not independently verified all the details in Wolff's book

Washington (CNN) White House press secretary Sarah Sanders faced questions Thursday for the second day in the row about President Donald Trump's mental fitness to serve as president.

"It's disgraceful and laughable. If he was unfit, he probably wouldn't be sitting there, wouldn't have defeated the most qualified group of candidates the Republican Party has ever seen," Sanders said, before praising Trump as an "incredibly strong" leader.

The questions come in the wake of the chaotic picture painted of the President in Michael Wolff's "Fire and Fury: Inside the Trump White House."

CNN has not independently verified all the details in Wolff's book.

Sanders disparaged the book, calling it "complete fantasy" and characterizing it as "sad" and "pathetic."

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