Which of the following is the most common serious immediate adverse effect of blood transfusion?

Bacterial contamination is recognized as one of the more serious immediate adverse effects of blood transfusion, especially in the context of sepsis or severe infections. When contaminated blood products are transfused, they can introduce pathogens into the recipient's bloodstream, leading to severe systemic reactions. These reactions can occur rapidly after the transfusion begins, often causing a spectrum of symptoms ranging from fever and chills to more severe manifestations like shock or organ failure.

The risk of bacterial contamination tends to increase with certain blood products, particularly platelet concentrates, as these products are stored at room temperature, which is conducive to bacterial growth. It underscores the importance of stringent screening processes and proper handling of blood products to mitigate such risks.

While other adverse effects like hemolytic transfusion reactions, urticaria, and hepatitis can occur with blood transfusions, they either present differently, have different causes, or, in the case of hepatitis, are not typically immediate effects. Hemolytic reactions often arise as a result of ABO incompatibility and may take time to manifest, while urticaria represents a mild allergic reaction that is less serious than bacterial sepsis. Therefore, understanding the mechanisms and risks associated with blood transfusion is essential for ensuring patient safety and effectively managing potential complications.