A 4-year-old boy develops fever, back and abdominal pain, and passage of dark brown urine five days after a viral upper respiratory infection. Hemoglobin drops to 6.5 g/dL with hemoglobinuria. Direct antiglobulin test is positive for C3 only. The responsible antibody is best characterized as:

• A IgG antibody with anti-P specificity showing biphasic thermal reactivity ✓
• B IgM antibody with anti-I specificity active at 4 degrees Celsius
• C IgG antibody directed against Rh antigens, optimally reactive at 37 degrees Celsius
• D IgA antibody against phosphatidylinositol glycan anchored proteins

Explanation

Paroxysmal cold hemoglobinuria is caused by the Donath-Landsteiner antibody, an IgG autoantibody against the P antigen. It binds red cells in the cold periphery, fixes complement, and then lyses cells when they return to core body temperature, hence biphasic reactivity. It classically affects children after viral infections and produces complement-only Coombs positivity. Anti-I IgM defines cold agglutinin disease, anti-Rh IgG defines warm AIHA, and GPI anchor defects describe PNH, which is Coombs negative.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.