✔ 最佳答案
crux /krʌks/ DJ /krʌks/ KK (google dictionary)
the most important or difficult part of a problem or an issue (難題或問題的)關鍵,最難點,癥結 noun singular the ~ (of sth)
「問題癥結」的英文是: The crux of the problem, "癥結"一詞跟"核心"有點不同, 因為"癥結"暗示問題有棘手之處