A 67-year-old woman presents to the clinic complaining of increasing shortness of breath on exertion. She has no prior cardiac or pulmonary history, and reports no symptoms of chest discomfort, cough, sputum production, orthopnea or peripheral edema. Her physical examination including vital signs, cardiac and pulmonary examinations are completely normal. Her CXR, ECG, and CBC are also normal. She then undergoes pulmonary function tests to evaluate her symptoms of dyspnea. The most prominent finding is a reduction of the ratio of FEV1/FVC with no reversibility when given inhaled salbutamol. Which of the following is the most likely diagnosis?