Question

What is grammatically wrong with this sentence, "A flare-up is when his joints hurt and stiffen up."

Answer 1

remove 'is' and replace it with 'occurs'

Answer 2

it is unclear what the pronoun 'his' refers to, as there is no person that precedes it. most would assume a 'flare-up' is not a person

Answer 3

Try: "A flare up is when one's joints hurt and stiffen up."

It was once acceptable to use "his" in papers, when referring to anyone non-specifically. Nowadays, to be gender-neutral, we use "one." If you're using this sentence in a school paper, that's the problem.

Answer 4

I would say the problem would be "is when his..." If i were were to rewrite this statement, i would instead say "A flare-up is characterized by his joints hurting and stiffening up". I also agree with Bevol, as there is usually a specific pronoun, or as BaconBitty points out, "one".