A 30-year-old, male smoker with a history of IV drug use and hepatitis C presented with insidious onset of exertional dyspnea, fatigue, and weight loss. He denied cough, wheeze, orthopnea, paroxysmal nocturnal dyspnea, fever, night sweats, or recent IV drug use. There was no relevant occupational or travel history, exposure to organic or inorganic allergens, or exposure to tuberculosis. The patient’s HIV status was unknown, and he was taking no prescribed medications. On examination, he was afebrile and acyanotic, with a BP of 102/52 mm Hg and pulse rate of 62 beats/min. Oxygen saturation on room air was 97% with a mild exertional desaturation to 90%. Peak flow was measured as 400 to 450 L/min. Chest auscultation revealed scattered fine rales without rhonchi. There were no features of central or peripheral edema, and the remainder of the physical examination was unremarkable.