Immunology USMLE Step 1 Practice Question
A 22-year-old man presents with episodic dark urine, fatigue, and dyspnea. Vital signs show HR 102/min, BP 118/76 mmHg, RR 18/min. Laboratory studies reveal hemoglobin 9.2 g/dL, elevated LDH at 850 U/L, and low haptoglobin. Flow cytometry demonstrates absent decay-accelerating factor (DAF) and CD59 on red blood cells due to PIGA mutation. Hemolysis occurs intravascularly. Platelet count and white blood cell count remain normal. Which mechanism best explains complement-mediated destruction of these erythrocytes?
Answer choices
- AIgE mediated mast cell degranulation
- BUncontrolled complement activation on the red cell surfaceCorrect answer
- CFailure to generate superoxide radicals in neutrophils
- DLoss of spectrin leading to membrane instability
- EAutoantibody mediated cold agglutination
- FCold agglutinin IgM complement fixation at low temperatures
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