Final answer:
When a patient develops acute onset confusion, the nurse's first action should be to assess oxygen saturation, as proper oxygenation is crucial for brain function and hypoxia can cause confusion.
Step-by-step explanation:
The first action a nurse should take when a patient develops acute onset confusion, especially following a serious injury like a fractured pelvis, is to assess the oxygen saturation. Oxygen saturation gives immediate information about the patient’s respiratory status and whether they are receiving adequate oxygenation. Proper oxygenation is vital for brain function, and hypoxia can lead to confusion and other neurological symptoms. Therefore, it is the most critical initial assessment. After ensuring there is no immediate life-threatening hypoxia, further examination, such as blood pressure measurement, orientation assessment, and pupil reaction checking would follow to understand the cause of confusion.