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》

This is the root cause of the problem.

这就是导致问题的原因所在。

youdao

Identify the root cause of the problem.

识别问题的根源。

youdao

Studies support the theory that there are several causes, instead of a single environmental cause or genetic problem.

VOA: special.2009.01.06

Many people said the problem with general levels of happiness or unhappiness rather, the cause is people have too high expectations.

很多人说,一般的快乐或不快乐水平，是由于人们有过高的期望。

哈佛公开课 - 幸福课课程节选

Don't just stop in the middle of the street because everyone is coming right behind, and it's going to cause a problem.

请不要停留在街道中央，因为街上人来人往，
你的停留是会惹麻烦的。

在纽约的感受 - SpeakingMax英语口语达人

And sure enough I saw the cause of the problem: my server was shut off.

FORBES: The Guy From Dell Thinks I'm A Dope...And He's Right

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

FORBES: Choosing the Right Training?

Depending on the cause of the problem, Boeing might be able to recoup any costs from suppliers.

BBC: Dreamliner: No fault found with Boeing 787 battery