A 64-year-old man with a history of smoking and chronic obstructive pulmonary disease (COPD) comes to his physician with an 8-month history of progressive shortness of breath, non-productive cough, and fatigue. He works from a home office in the information technology field. On physical examination, he has bibasal inspiratory crackles and clubbing in both hands.

Chest x-ray shows bilateral reticulonodular shadows and traction bronchiectasis. High-resolution CT scan shows patchy peripheral reticular abnormalities, ground glass opacities, subpleural honeycombing, and traction bronchiectasis. The lower lung fields appear to be more involved than the upper lung fields. Lung function tests show the following:

• Total lung capacity: 6.0 L (decreased)

• Functional residual capacity: 2.9 L (decreased)

• Residual volume: 1.9 L (decreased)

• Forced expiratory volume/Forced vital capacity: 70% of predicted (low normal)

• Diffusing capacity for carbon monoxide (DLCO): 55% of predicted (decreased)

Which of the following is the most likely diagnosis?