Ailing

crux \KRUHKS\, noun;
plural cruxes, also cruces \KROO-seez\:

1. The basic, central, or critical point or feature.
2. Anything that is very puzzling or difficult to explain or solve.

Ah, and now it was out of the bag, O'Kane thought, the crux of the matter.
-- T. Coraghessan Boyle, Riven Rock

The crux of the problem, he said, was "the real disparity between our views and those of the people with whom we have to deal and who are in control."
-- John Bierman and Colin Smith, Fire in the Night

Members of an audience cannot stop the actors and puzzle over some difficult expression, as they can when reading the play. The action sweeps you past the crux, which is at once forgotten because you need to keep up with what is being said, not lose the plot by meditating on what has passed.
-- Frank Kermode, Shakespeare's Language

Crux is from Latin crux, "cross, torment, trouble."