the root of the problem

root of the problem, cause of the problem, underlying issue, core issue, main problem, primary cause, fundamental cause, source of the problem, problem analysis, problem investigation, problem identification, problem solving, problem resolution, problem management, issue analysis, issue investigation, issue identification, issue resolution, issue management, root cause, root cause analysis, root cause identification, root cause investigation, root cause resolution, root cause management, problem origin, problem factor, problem reason, issue origin, issue factor, issue reason, problem basis, problem foundation, issue basis, issue foundation, problem source, issue source, underlying cause, fundamental problem, central issue, core cause, key problem, main issue