Final answer:
In an emergency following a motor vehicle accident where the patient's blood type is unknown, O-negative blood is used for transfusion due to its universal compatibility and minimized risk of adverse reaction.
Step-by-step explanation:
Following a motor vehicle accident, a patient with multiple traumatic injuries necessitating urgent care would receive O-negative blood during a transfusion. This is because O-negative blood is considered universal and can be safely given to patients of any blood type during an emergency when there is no time to determine the patient's blood type. Since O-negative blood cells do not have A or B antigens on the surface and lack the Rh factor, they are less likely to cause a reaction with the recipient's immune system.