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?

Primary pulmonary Langerhans cell histiocytosis (PLCH), also called eosinophilic granuloma of the lung, pulmonary Langerhans cell granulomatosis, and pulmonary histiocytosis X, is an uncommon interstitial lung disease that primarily affects young adults. There is a coarse, reticular pattern in the whole lung, somewhat more prominent in the upper lobes, suggesting a honeycomb appearance. It is the density here that is abnormal and not the lucency. The next most appropriate test would be a better definition of the lung findings with a high-resolution CT scan. The other more invasive investigations may be more appropriate after the CT.