<s>[INST] context: Patients with chronic granulomatous disease have defective functioning of phagocytic neutrophils and monocytes due to an inability to produce hydrogen peroxide. That is, their phagocytic cells have a decreased oxidative or respiratory burst. The most common cause of chronic granulomatous disease is defective NADPH oxidase, which is an enzyme on the membrane of lysosomes that conves O2 to superoxide and stimulates oxygen burst. This deficiency results in recurrent infections with catalasepositive organisms, such as S. aureus. The classic form of chronic granulomatous disease usually affects boys and causes death before the age of 10. Key findings in chronic granulomatous disease include lymphadenitis, hepatosplenomegaly, eczematoid dermatitis, pulmonary infiltrates that are associated with hypergammaglobulinemia, and defective ability of neutrophils to kill bacteria. A defect in the enzyme adenosine deaminase (ADA) is seen in the autosomal recessive (Swiss) form of severe combined immunodeficiency disease (SCID), while a defect in the IL-2 receptor is seen in the X-linked recessive form of SCID. A developmental defect at the pre-B stage is seen in X-linked agammaglobulinemia of Bruton, while developmental failure of pharyngeal pouches 3 and 4 is characteristic of DiGeorge's syndrome. [/INST] question: In an evaluation of an 8-year-old boy who has had recurrent infections since the first year of life, findings include enlargement of the liver and spleen, lymph node inflammation, and a superficial dermatitis resembling eczema. Microscopic examination of a series of peripheral blood smears taken during the course of a staphylococcal infection indicates that the bactericidal capacity of the boy's neutrophils is impaired or absent. What is the most likely cause of this child's illness? | options: A: Defect in the enzyme NADPH oxidase B: Defect in the enzyme adenosine deaminase (ADA) C: Defect in the IL-2 receptor D: Developmental defect at the pre-B stage | answer: A </s>