A 6-year-old boy has recurrent pyogenic infections with catalase-positive organisms (Staphylococcus aureus, Aspergillus, Klebsiella) since infancy, with lymphadenitis and hepatosplenomegaly. NBT (nitroblue tetrazolium) test is negative. The defect is in which enzyme complex?

• A Myeloperoxidase in azurophilic granules — myeloperoxidase deficiency
• B NADPH oxidase (phox complex) in neutrophils — chronic granulomatous disease ✓
• C Adenosine deaminase (ADA) in lymphocytes — ADA-SCID
• D Actin polymerisation in neutrophils — LAD (leukocyte adhesion deficiency)

Explanation

A negative NBT test (failure to reduce yellow NBT to blue formazan) indicates absent NADPH oxidase activity — the hallmark of chronic granulomatous disease (CGD). CGD neutrophils can ingest but cannot kill catalase-positive organisms that destroy their own H2O2, removing the substrate neutrophils use when NADPH oxidase is defective. Myeloperoxidase deficiency rarely causes severe infections; ADA-SCID affects T and B cells; LAD causes absent CD18.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.