A 3-year-old boy develops dark urine and pallor one week after a viral upper respiratory infection. Hemoglobin is 6 g/dL, plasma hemoglobin is elevated, and haptoglobin is undetectable. Direct antiglobulin test is positive for complement (C3d) only. Which antibody specificity is characteristic of this condition?

• A IgM anti-I agglutinating optimally at 4 degrees C
• B IgG anti-P with biphasic thermal amplitude (Donath-Landsteiner) ✓
• C IgG anti-Rh with wide thermal amplitude
• D IgA anti-i binding at 37 degrees C

Explanation

This is paroxysmal cold hemoglobinuria. The Donath-Landsteiner antibody is an IgG directed against the P antigen that binds red cells in cool peripheral circulation and fixes complement; rewarming in central vessels completes lysis, giving the biphasic behavior and dramatic intravascular hemolysis. It occurs in children after viral illness and shows C3d-only positivity. Anti-I IgM describes classic cold agglutinin disease, which causes acrocyanosis and chronic hemolysis rather than postviral hemoglobinuria.

Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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