the crux
noun [ S ] uk/ˈkrʌks/ us/ˈkrʌks/
关键;核心;症结
the most important or serious part of a matter, problem, or argument
The crux of the country's economic problems is its foreign debt. 外债是该国经济问题的症结所在。
The issue of an arms embargo will be at the crux of the negotiations in Geneva. 武器禁运将是日内瓦谈判的核心议题。