Final answer:
In emergency situations where a trauma patient’s blood type is unknown and they require immediate transfusion, type O negative blood is used because it is the universal donor and can be given to patients of any blood type without severe reactions.
Step-by-step explanation:
When treating an unstable trauma patient as in a motor vehicle accident with severe bleeding, and there is no time to determine the patient's blood type, type O negative blood will be transfused. Type O negative blood is the universal donor, which means it can be received by individuals of any blood type without causing a transfusion reaction that can further complicate the patient's critical condition. This is essential in emergencies when there isn’t enough time to conduct a blood type test, and the patient requires an immediate blood transfusion to survive.