A 56-year-old man complains of fatigue, dyspnea on exertion and palpitations. He has had a murmur since chi- ldhood. Examination reveals a lift at the left sternal border, split Si, and fixed splitting of S2. There is a grade 3/6 midsystolic pulmonic murmur and a 1/6 middiastolic tricuspid murmur at the lower left sternal border. Chest X-ray shows right ventricular enlargement and prominent pulmonary arteries. ECG demonstrates atrial fibrillation with a right bundle branch block. The most likely diagnosis is: