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