LONDON — Parsing what is real and what is not isn’t always easy in the age of social media and fake news, as J. K. Rowling, arguably the world’s most popular and beloved fantasy writer, has discovered.

Ms. Rowling, author of the “Harry Potter” books, provoked a barrage of criticism on social media and beyond after accusing President Trump of willfully ignoring Montgomery Weer, a 3-year-old boy with spina bifida who uses a wheelchair, during an event late last month at the White House.

“How stunning, and how horrible, that Trump cannot bring himself to shake the hand of a small boy who only wanted to touch the President,” wrote the author, a canny Twitter user who has adeptly fended off critics with the aplomb of a Hogwarts magic spell.

“My mother used a wheelchair. I witnessed people uncomfortable around her disability, but if they had a shred of decency they got over it,” she wrote on Twitter in comments that have since been deleted. “So, yes, that clip of Trump looking deliberately over a disabled child’s head, ignoring his outstretched hand, has touched me on the raw.”