Final answer:
The most serious complication after a blood transfusion is an incompatible transfusion reaction, which can lead to severe symptoms and even death.
Step-by-step explanation:
The most serious complication that can occur within 6-12 hours after a blood transfusion is an incompatible transfusion reaction. In this type of reaction, if a person receives blood that is incompatible with their own blood type, their immune system will respond by attacking and destroying the transfused red blood cells (RBCs). This can lead to a cascade of events including fever, chills, itching, hives, difficulty breathing, red or brown urine, and low blood pressure. In severe cases, incompatible transfusion reactions can result in shock, multi-organ failure, and death.