A nurse in an emergency department is caring for a client who has a closed head injury. Which of the following actions should the nurse take first?

The Glasgow Coma Scale (GCS) is a rapid neurological assessment tool that evaluates eye opening, verbal response, and motor response — it is the first priority to establish baseline neurological status and guide further care. MRI is time-consuming and not the first step in an acute head injury. An indwelling catheter is important for fluid monitoring but not the first priority. Mannitol (osmotic diuretic for ICP reduction) is an intervention based on assessment findings, not the first action.