Learner's definition of CRUX
the crux
: the most important part of something (such as a problem, issue, puzzle, etc.)（问题、疑难等的）关键，症结—
usually + of -
The crux of the matter is that people are afraid of change.问题的关键是人们害怕改变。
-
It's taken a while to get to the crux of the problem, but I think I finally understand it.找到问题的症结颇费周折，但是我想我终于搞明白了。