What would you say was the root cause of the problem?

你认为问题的根源是什么？

《牛津词典》

Debugging a problem involves locating the source of the problem using a debugger (or any other tool) and using the source code to track the root cause of the problem.

调试一个问题涉及到：使用调试器(或任何其它工具)找到问题的根源以及使用源代码来跟踪导致问题的根源。

youdao

Business performance often suffers as cross-functional teams struggle to identify and fix the root cause of the problem.

在职能交叉团队努力确定和修正问题的根本原因的过程中，软件的商业性能往往很差。

youdao

So rather than blindly following the mantra, analyze the root cause of the problem at hand.

FORBES: Choosing the Right Training?

Regardless of who you point the finger at, the root cause of the problem seems to be the same.

FORBES: Winning Takes Care of Everything

The issue is a concern to Shaw and he suggests that over-training could be the root cause of the problem.

BBC: Shaw questions gym-based approach