A 40-year-old woman comes to the physician for a follow-up examination. She has a 20-year history of multiple respiratory tract infections, including four pneumonias during the past 15 years and three episodes of sinusitis during the past 10 years. She typically requires antibiotic therapy for bronchitis 2-3 times annually. Vitals are normal. Cardiopulmonary examination shows coarse breath sounds bilaterally and a loud P2. There is no clubbing or edema. In addition to pulmonary function testing, which of the following is the next best step in diagnosis?
Ascertainment of CD4+ T cell count
Ascertainment of serum complement concentrations
Ascertainment of serum immunoglobulin concentrations
Throat culture
Ventilation-perfusion scan
