crux of the matter
Also, heart of the matter. The basic, central or critical point of an issue. For example, In this trial the bloodstains represent the crux of the matter, or We think the second clause is the heart of the matter. Although crux is Latin for “cross,” in English it means “difficulty” or “puzzle,” and it is from the latter that this expression is thought to be derived. The variant employs heart in the sense of “a vital part” (as it is in the body). The first term dates from the late 1800s, the variant from the early 1500s.