A 35-year-old man is evaluated in the clinic for symptoms of shortness of breath. He reports no other lung or heart disease. He smokes half pack a day for the past 10 years. On examination, his JVP is at 2 cm, heart sounds normal, and lungs are clear. A CXR shows hyperinflation and increased lucency of the lung fields. A chest CT reveals bullae and emphysematous changes in the lower lobes, while pulmonary function tests show an FEV1/FVC ratio of < 70%. Evaluation of his family reveals other affected individuals. Which of the following is the most likely diagnosis?