Trump told reporters at the White House that the traditional presidential check-up had "better go well."

"I think it's going to go very well," the president said. "I'll be very surprised if it doesn't."

"It better go well, otherwise the stock market will not be happy," he joked later.

Trump's comments come one day before he will undergo a physical exam at Walter Reed National Military Medical Center.

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Dr. Ronny Jackson, the White House physician, will conduct the exam, and the White House has said it will release the results.

Jackson will take the unusual step of answering questions about the president's exam in the White House briefing room next Tuesday , according to press secretary Sarah Huckabee Sanders. Trump, 71, is the oldest man to be elected president. He frequently boasted about his physical stamina during the 2016 presidential campaign.

The exam comes amid persistent questions about Trump's mental fitness, which were sparked by a new book that paints him as ill-equipped to occupy the Oval Office.

Trump has dismissed those questions, calling himself a "very stable genius."