Immunodeficiencies Quiz: Ace Your Exams

A 2-year-old girl is brought to her primary pediatrician for evaluation of recurrent infections. In the past year, the patient has had two bouts of lobar pneumonia and several episodes of otitis media. Since birth, she has had multiple skin and soft tissue infections that resolve with oral antibiotics. She was born at 38 weeks gestation to a 28-year-old healthy mother. The patient’s umbilical cord separation was delayed by forty days after birth. She eats and drinks normally and has achieved all developmental milestones. Family history is unremarkable. Vitals are within normal limits. On physical exam, a 2 cm x 2 cm well-circumscribed fluctuant lesion is noted. Incision and drainage of this skin lesion expresses a moderate amount of sero-sanguinous fluid. Follow up fluid culture grows Staphylococcus aureus, and flow cytometry studies reveal absence of CD18 antigens on the surface of the patient’s leukocytes.  Which of the following immunoprotective mechanisms is defective based on this patient’s clinical presentation?