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?

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. 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.