Mary Hanson, a 2-year old, was brought to the doctor's office by her mother after she discovered two swollen and painful lumps in Mary's groin. Staphylococcus aureus was cultured from phagocytic cells obtained from the affected lesions and granulomas were noted. Additional tests revealed that neutrophils from Mary do not activate a respiratory burst after phagocytosis. The most likely protein defect would be