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?

A. Mannitol is used to decrease intracranial pressure but should only be given after assessing the client's neurological status. B. An MRI can help diagnose brain injuries but is not the priority before assessing the client's condition. C. The Glasgow Coma Scale (GCS) helps assess the severity of brain injury and is crucial in determining the immediate needs of the client. D. A urinary catheter may be needed but is not the priority over neurological assessment.