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

Question

anonymous · Answer

Is their a problem? Bcause it just sounds a little out of place...I guess. Might just be my bad grammatical error capturing.

anonymous · Answer

"When he has a flare-up, his joints hurt and stiffen up." might be better. 

The beginning is wrong/awkward because it's generalizing a thing that happens to lots of people (the flare up) to him only and then listing symptoms. 

I find 'stiffen up' in the original context awkward, but not wrong.

anonymous · Answer

I try to avoid "is when" constructions and any passive constructions in general. Depending on what you intend to convey, the revision from clstal could work well. If you intend to define a flare-up you might want to revise as "A flare-up causes joints to stiffen and hurt."