A 5-year-old boy presents with recurrent catalase-positive bacterial infections (Staphylococcus, Aspergillus, Klebsiella) since infancy. NBT (nitroblue tetrazolium) test is negative. The underlying defect is in which specific pathway?
- A Myeloperoxidase-halide system for bacterial killing
- B Lysosomal microtubule function for phagolysosome fusion
- C Leukotriene B4 production for neutrophil chemotaxis
- D NADPH oxidase complex generating superoxide radicals (respiratory burst) ✓
Explanation
The clinical scenario describes Chronic Granulomatous Disease (CGD), where NADPH oxidase (phox) complex is defective. Normally, NADPH oxidase catalyses the production of superoxide (O2−), which is then converted to H2O2 and other reactive oxygen species (respiratory burst) that kill catalase-positive organisms. Catalase-positive organisms (Staphylococcus, Aspergillus, Klebsiella, Serratia, Nocardia) neutralise the small amounts of H2O2 they themselves produce, so they survive in CGD neutrophils. The NBT test is negative because neutrophils cannot reduce the dye (which requires superoxide). Chediak-Higashi syndrome involves microtubule dysfunction for phagolysosome fusion, not respiratory burst.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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