Five days after receiving high dose intravenous penicillin for pneumonia, a 40-year-old man develops hemoglobinuria and a hemoglobin drop from 12 to 6 g/dL. Direct antiglobulin test is positive. Which mechanism accounts for the hemolysis?
- A Drug binds the red cell surface and IgG targets the drug membrane complex (hapten type) ✓
- B Drug induces IgM that binds I antigen optimally at 4 degrees C
- C Immune complexes form in plasma and deposit nonspecifically on red cells
- D Complement independent splenic clearance of drug coated platelets
Correct answer: A. Drug binds the red cell surface and IgG targets the drug membrane complex (hapten type)
Explanation
Penicillin acts as a hapten: at high doses it coats the red cell membrane firmly, and IgG directed against the drug membrane complex causes extravascular hemolysis with a positive direct Coombs test. Option C describes the classic innocent bystander mechanism, typical of quinidine or cephalosporins via IgM and complement. Cold IgM against the I antigen is idiopathic or infectious cold agglutinin disease, and option D describes immune thrombocytopenia, not anemia.
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.