A 14-year-old boy presents with recurrent skin abscesses caused by Staphylococcus aureus and Serratia marcescens. He has elevated IgG, normal lymphocyte counts and normal complement levels. His nitroblue tetrazolium (NBT) test shows 2% positive cells (normal >95%). Which of the following best explains the defect in this patient?
- A Absence of NADPH oxidase activity in phagocytes ✓
- B Deficiency of myeloperoxidase in neutrophils
- C Impaired T-cell receptor signalling
- D Loss of CD18 integrin expression
Explanation
An NBT test result of 2% positive cells indicates a near-complete failure of oxidative burst, diagnostic of chronic granulomatous disease (CGD) caused by deficiency of NADPH oxidase (phox complex). This leads to inability to kill catalase-positive organisms such as Staphylococcus aureus and Serratia, while IgG levels remain normal since B-cell function is intact. Myeloperoxidase deficiency causes a milder phenotype with partial killing and normal NBT. CD18 deficiency causes leukocyte adhesion deficiency, characterised by absence of pus formation and delayed umbilical cord separation.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.