Crux

Lace edging
Originally uploaded by darklordspet

According to Merriam-Webster, crux is:
1 : a puzzling or difficult problem : an unsolved question
2 : an essential point requiring resolution or resolving an outcome
3 : a main or central feature (as of an argument)
Etymology: Latin cruc-, crux cross, torture

The lace edging is essential to the completion of the shawl, and it's torturing me.