Which statement about Paroxysmal nocturnal hemoglobinuria is true?

• A. It is caused by a complete absence of hemoglobin
• B. It is primarily an extravascular hemolysis disorder
• C. It is due to deficiency of CD4+ T cells
• D. It is due to a deficiency of GPI-anchored proteins CD55 and CD59 on hematopoietic cells

Answer: (D)

Paroxysmal nocturnal hemoglobinuria arises from a defect in the GPI anchor synthesis that leaves blood cells lacking GPI-anchored proteins, especially the complement regulators CD55 and CD59. Without these protective proteins, the complement system can attack the cell membranes, leading predominantly to intravascular hemolysis as the membrane attack complex forms on red blood cells. This explains episodes of hemoglobinemia/hemoglobinuria and the tendency toward anemia, and it also helps account for thrombosis risk related to platelet involvement. It is not caused by a deficiency of CD4+ T cells, and it’s not a simple absence of hemoglobin; the issue is loss of surface protection from complement due to missing GPI-anchored proteins.