A 71-year-old man is seen with low-grade fever, generalized malaise, and a run-down feeling. He has lost weight and shows stigmata of chronic illness. There is no history of occupational exposure. On physical examination, vital signs are as follows: pulse 110 bpm; temperature 99degF; respirations 19/min; blood pressure 90/60 mm Hg. On exam, the man is frail and appears cachectic with temporal wasting. Other aspects of his physical exam are unremarkable. Laboratory data: Hb 10 g/dL; Hct 30%; MCV 90; WBCs 3000/uL; differential normal; BUN 19 mg/dL; creatinine 1.0 mg/dL; sodium 129 mEq/L; potassium 5.0 mEq/L; ABGs (RA): pH 7.42, PCO2 35 mm Hg, PO2 58 mm Hg. Spirometry: FVC 60% of predicted; FEV1 60% of predicted. PPD skin test is negative (0 mm); induced sputum for AFB smear is negative. Chest radiograph is shown below.What is the most likely diagnosis?