keyproblem
nounUK /ˈkiːˌprɒbləm/
US /ˈkiːˌprɑːbləm/
The most important problem that needs to be fixed first.
The key problem is that we don't have enough water.
OTHER MEANINGS
2. The main question in a test or puzzle that must be answered.
Read the key problem carefully before you start solving it.
3. The most important difficulty in a plan or project.
The key problem with the plan is that it costs too much.
4. A central trouble that makes other small troubles happen.
Fixing the key problem stopped many smaller problems too.