A 41-year-old woman comes to the physician for a routine health maintenance examination. She has a 20-year history of multiple respiratory tract infections, including 4 pneumonias and 5 episodes of sinusitis. She usually requires antibiotic therapy for bronchitis 3-4 times per year. Vitals are within normal limits. Physical examination shows vitiligo on the hands. Auscultation of the chest shows a loud P2. Course breath sounds are heard bilaterally. Which of the following is the most appropriate next step in diagnosis?
Measurement of CD4+ T cell count
Measurement of serum complement concentrations
Measurement of serum immunoglobulin concentrations
Rapid Strep test
Spiral CT
