In paroxysmal nocturnal hemoglobinuria (PNH), the molecular defect is a somatic mutation in PIG-A gene, leading to absence of GPI-anchored complement regulatory proteins. Which two proteins, when absent, make red cells vulnerable to complement-mediated lysis?

• A Factor H and Factor I
• B CD55 (DAF) and CD59 (protectin) ✓
• C C1-inhibitor and C4b-binding protein
• D Properdin and MASP-2

Explanation

PNH red cells lack GPI-anchored CD55 (decay-accelerating factor, which degrades C3/C5 convertases) and CD59 (protectin/MIRL, which blocks the terminal membrane attack complex). Absence of both leaves red cells exquisitely sensitive to spontaneous complement activation, particularly under acidic conditions at night. Factor H and I are fluid-phase regulators, not GPI-anchored.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.