Final answer:
Ensuring that the recipient's blood type matches the donor blood is the most crucial nursing responsibility during a blood transfusion to prevent potentially fatal reactions.
Step-by-step explanation:
The most important nursing responsibility during a blood transfusion is ensuring that the recipient's blood type matches the donor blood. This is critical since incompatible blood types can lead to hemolytic transfusion reactions, which are potentially fatal. Therefore, cross matching must be performed to determine compatibility.
In emergency situations where there is severe hemorrhage and no time to determine the blood type, type O blood can be used as it is considered the universal donor.