A 21-year-old man is seen in the clinic for assessment of a nonproductive cough, shortness of breath, and chest pain which changes with breathing. He also complains of pain in the left arm. On physical examination, there is tenderness over the left shoulder, heart sounds are normal, and the lungs are clear. A CXR reveals a lytic lesion in the left humerus and reticulonodular opacities in the upper and middle lobes (Figure below). The eosinophil count is normal. Which of the following is the most appropriate initial diagnostic test?