crux 
/ˈkrʌks/ noun
/ˈkrʌks/ noun Meaning:
the crux
: the most important part of something (such as a problem, issue, puzzle, etc.) — usually + of ▪ The crux of the matter is that people are afraid of change. ▪ It's taken a while to get to the crux of the problem, but I think I finally understand it.
