A 3-year-old girl develops cola-colored urine and pallor one week after a viral illness. Hb is 6.5 g/dL with hemoglobinemia. Direct antiglobulin test is positive for C3d only. The antibody classically responsible fixes complement at 4 degrees C and causes hemolysis as cells rewarm to 37 degrees C. This antibody is:
- A IgG anti-Rh directed against D antigen
- B IgM anti-I directed against adult I antigen
- C IgG biphasic hemolysin directed against P antigen (Donath-Landsteiner antibody) ✓
- D IgA anti-i directed against fetal i antigen
Correct answer: C. IgG biphasic hemolysin directed against P antigen (Donath-Landsteiner antibody)
Explanation
Paroxysmal cold hemoglobinuria is mediated by the Donath-Landsteiner antibody, an IgG that binds the P antigen in the cold, fixes complement, and lyses cells on rewarming, hence the term biphasic hemolysin. The DAT shows C3 alone because the antibody elutes off at warm temperature. IgM anti-I belongs to cold agglutinin disease of adults with Mycoplasma or EBV infection, which causes agglutination and extravascular clearance rather than dramatic hemoglobinuria.
Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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