During recent public appearances, former president Donald Trump displayed mysterious red marks on his hand.

The marks could be spotted on his thumb, index finger and palm as he waved to cameras. This led to speculations over the cause, and according to some, the issue with Trump’s hand could be a result of a fall, a hand herpes, or even ketchup.

Dr. Joshua Zeichner, an associate professor of dermatology at the Mount Sinai Hospital, was questioned of the potential reasons for the appearance of the red marks. Speaking to Business Insider, he explained, “Rashes on the hands can be caused by a variety of factors, ranging from infections to inflammatory conditions to external exposures.”