A 73 year-old man presents with progressive dyspnea on exertion over the past one year. He reports a dry cough but no wheezes, sputum production, fevers or hemoptysis. He is a life-long non-smoker and worked as a lawyer until retiring 3 years ago. His pulmonary function testing is as follows. What is the most probable diagnosis? Pre-Bronchodilator (BD)TestActualPredicted% PredictedFVC (L)1.574.4635FEV1 (L)1 283.3938FEV1/FVC (%)8276 FRC1 733.8045RV (L)1.122 5943TLC (L)2.706.4542RV/TLC (%)4142 DLCO corr5.0631.6416