参见：症瘕[zhēngjiǎ]

问题难于解决的关键crux

• 找到失败的症结所在

  find the crucial reason behind the failure

• 问题不得解决的症结显然不在这里。

  Clearly, we're not going to find the key reason why the problem can't be solved here.

• 这就是症结所在。

  Therein lies the crux of the problem.