It was B, which matched their sizing chart for my size and weight.

My assumption is that mesh was warn out and sagging a bit too much causing the pinch. If for some reason I need a new chair I'm going to see if I can try a new model to see if that was the problem.

I wouldn't be sad though if I had to keep it, it's a nit from me. It's a great chair and deserves all the hype.