A patient with recurrent bacterial infections has neutrophils that show impaired killing of catalase-positive organisms despite normal phagocytosis. The NBT (nitroblue tetrazolium) test is negative. Which enzyme is deficient?
- A Myeloperoxidase
- B NADPH oxidase (gp91phox subunit) ✓
- C Lysozyme
- D Elastase
Correct answer: B. NADPH oxidase (gp91phox subunit)
Explanation
This is chronic granulomatous disease (CGD) caused by mutations in NADPH oxidase components (most commonly gp91phox, X-linked). Defective NADPH oxidase cannot generate superoxide (respiratory burst), so the NBT test is negative. Catalase-positive organisms (Staphylococcus aureus, Aspergillus) are particularly dangerous because they destroy the limited H₂O₂ the bacteria themselves generate, leaving no oxidants for killing.
Reference: Robbins & Cotran Pathologic Basis of Disease, 10th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.