Final answer:
In a mass casualty explosion, the nurse should first apply direct pressure to a large, open occipital wound to control bleeding before any other steps like cleaning or dressing the wound.
Step-by-step explanation:
When dealing with a mass casualty explosion and triaging a client with a large, open occipital wound, the nurse must act with urgency and prioritize actions based on the goal of preserving life. The most immediate concern is to control hemorrhage and prevent further injury or infection. In such a critical scenario, the first step should be to apply direct pressure to the wound to control bleeding, which is a life-threatening emergency. The use of advanced imaging techniques such as MRI or CT scans to localize the injury would be ideal but is not practical in the acute phase on the scene of the incident. After controlling the bleeding, cleaning the wound and dressing it with sterile materials would follow as next steps. It is not the time for administering pain medication or cleaning with antiseptic solution before ensuring that the bleeding is controlled.