Inflammation and Immune Pathology USMLE Step 1 Practice Question
A 34-year-old man presents to the emergency department with a 3-day history of jaundice, dark urine, and exertional dyspnea. He denies recent infections, new medications, or transfusions. Vital signs: BP 128/82 mmHg, HR 102/min, RR 20/min, temperature 37.2°C, SpO2 98% on room air. Laboratory studies show: hemoglobin 7.2 g/dL (baseline unknown), reticulocyte count 12%, total bilirubin 6.2 mg/dL with indirect bilirubin 4.8 mg/dL, haptoglobin <10 mg/dL, LDH 890 U/L, direct antiglobulin test (Coombs) positive for IgG but negative for complement. Peripheral smear shows spherocytes. Which of the following best explains the mechanism of red blood cell destruction in this patient?
Answer choices
- AIgM autoantibodies binding RBCs and activating the classical complement pathway with intravascular hemolysis
- BIgG autoantibodies coating RBCs with subsequent Fc-receptor-mediated extravascular hemolysis in the spleenCorrect answer
- CCytotoxic T lymphocytes directly infiltrating bone marrow and destroying erythroid precursor cells
- DIgE-mediated mast cell degranulation triggering immune complex deposition on RBC membranes
- ECold-reactive IgM antibodies causing RBC agglutination and complement-mediated intravascular hemolysis
- FNatural killer cell recognition of altered self-antigens on RBCs independent of antibody binding
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