An elderly patient is receiving a blood transfusion for anemia due to myelodysplastic syndrome (MDS). He was diagnosed with MDS 2 years ago and over the past 6 months has required blood transfusions every 6 weeks for symptomatic anemia. His past medical history also includes hypertension, Type 2 diabetes, and coronary artery disease. Halfway through the transfusion of the second unit of packed red blood cells he develops tachypnea, lumbar pain, tachycardia, and nausea. Which of the following is the most likely explanation?

Intravascular hemolysis from blood transfusion is usually due to ABO incompatibility, often from human error. Symptoms of intravascular hemolysis include flushing, pain at the infusion site, chest or back pain, restlessness, anxiety, nausea, and diarrhea. Signs include fever and chills, shock, and renal failure. In comatose patients, hemoglobinuria or bleeding from disseminated intravascular coagulation can be the first sign. Management is supportive. Acute hemolysis can also result from antibodies directed against other RBC antigens such as Rh, Kell, or Duffy.