Main Cause
Primary Cause
noun
The main cause is the main cause or the primary cause.
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.
インフレの主因って何だと思いますか？
What do you think the main cause of inflation is?
主因はウルトラプロセスフードですよ。
The main culprit is ultra-processed food.
ニアミスの主因は、この電車がスピードを上げるべきではないタイミングで上げたことです。
The main cause behind the near-miss was that the train sped up when it wasn't supposed to.