A 55-year-old male has a 3-month history of chronic shortness of breath and dyspnea on exertion. His physical examination is unremarkable except for 1+ ankle edema bilaterally and a soft systolic murmur. A stress echocardiogram is normal. Pulmonary function tests are normal except for a low diffusing capacity of the lung for carbon monoxide (DLCO).
Which one of the following conditions should be considered in this patient?

A) Chronic pulmonary thromboembolism
B) Emphysema
C) Interstitial lung disease
D) Asthma
E) Hypersensitivity pneumonitis