root, principle cause (of a problem) / prime culprit, instigator, leader of a criminal gang

question, problem on homework etc. / a question; a matter (of time etc.) / a problem (something that needs to be dealt with); a trouble / an issue; a matter

(informal) no problem

unthinkable, out of the question

practical question (problem)

matter of life or death / (idiomatic) an extremely urgent matter

viewing as a problem

to view as a problem

set of problems

problem; point at issue