Crux (noun)

1. The most important or central point of something.
2. A difficult problem or difficulty that is the central part of something.

Origin:

Late 16th century: from latin, "cross", "crosspiece", used in the sense "pivotal point".

Examples:

1. The crux of the matter was how to find a solution.
2. The crux of his argument was that the policy was flawed.
3. The crux of the issue was how to balance the budget.
4. The crux of her proposal was to increase funding for education.
5. The crux of the debate was the morality of the death penalty.