nub

1. N-SING The nub of a situation, problem, or argument is the central and most basic part of it. 中心; 核心['the' N, usu N 'of' n]

That, I think, is the nub of the problem. 我想那才是问题的核心。

Here we reach the nub of the argument. 现在我们谈到了争论的关键部分。