Immunology USMLE Step 1 Practice Question
A 37-year-old woman receives a red blood cell transfusion following postpartum hemorrhage. Within minutes, she develops fever (39.2°C), chills, flank pain, and dark urine. Vital signs show hypotension (88/54 mmHg), tachycardia (118/min), tachypnea (22/min), and SpO2 97% on room air. Laboratory studies reveal elevated creatinine (2.1 mg/dL) and positive direct antiglobulin test. Hemolysis markers are significantly elevated. She denies prior transfusions. Which immune mechanism most likely explains this acute transfusion reaction?
Answer choices
- AAntibodies stimulate acetylcholine receptors causing neuromuscular weakness
- BPreformed IgM antibodies against donor erythrocyte antigens activate complementCorrect answer
- CRecipient IgE antibodies bind donor plasma proteins and trigger mast cell degranulation
- DImmune complex deposition in small vessels with neutrophil recruitment
- EDonor T lymphocytes attack recipient epithelial tissues
- FRecipient IgG sensitizing donor RBCs for extravascular splenic destruction
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