Detroit, Michigan - Donald Trump is so proud of a cognitive assessment he took while president that he boasted about it in a speech Saturday, while attacking what he claims is President Joe Biden's lack of mental acuity.

The only problem: he then immediately got confused on the name of the doctor who oversaw the test.

Trump extolled his powers of mental recall in a speech to hard-right allies in Detroit, challenging Biden to take the same cognitive exam he says he underwent in 2018 with then-White House physician Ronny Jackson.

However, he immediately flubbed the name of the former official, who is now one of his loyal supporters in Congress.

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"Doc Ronny Johnson," said Trump, who turned 78 on Friday.

"Does everyone know Ronny Johnson, congressman from Texas?" Trump continued. "He said I was the healthiest president, he feels, in history, so I liked him very much."

As part of his attack on what he says is the 81-year-old Biden's declining mental acuity, Trump told the laughing crowd in Detroit that the president had been lost while at the G7 in Italy, turning "to look at trees," while other world leaders were looking at a parachutist who'd just landed in an aerial display.

The moment Trump referred to is based on a video that has gone viral in right-wing media and Republican circles.

However, the video is misleading because it has been edited, leaving out footage that shows Biden turning away from the other G7 leaders to gesture to a different skydiver who had just landed to his left.