From Longman Dictionary of Contemporary Englishcruxcrux /krʌks/ noun → the crux
Examples from the Corpus
crux• In the first place, v. 21b is a notorious crux of interpretation.• Whether we can get funding or not is the crux.• Finding a cheap source of energy is really the crux of the matter.• The women kicked off on fairly straight forward F7a, but the last move was the crux.• He didn't much care; that was the crux of the matter.• The crux of the court case is whether consumers deserve a refund.• The crux of the matter is that attitudes on the character and scope of planning have changed.Origin crux (1600-1700) Latin → CROSS2 