A 72-year-old male patient with a 40-year history of smoking, recurrent cough and sputum production for 30 years, shortness of breath on exertion for 13 years, and bilateral lower extremity edema for 5 years. Echocardiogram (ECHO) shows right ventricular hypertrophy and widened right ventricular outflow tract. What is the most common cause of chronic cor pulmonale?