A 45-year-old male presents with a 2-year history of chest tightness and palpitations on exertion, with poor response to nitroglycerin. BP: 130/80 mmHg (17.3/10.7 kPa). Physical exam reveals cardiomegaly, normal IgE, heart rate 80 bpm with regular rhythm, audible S4 at the apex, and a systolic ejection murmur at the left 3rd-4th intercostal space. Lungs clear, abdomen unremarkable. Echocardiogram shows normal left ventricular cavity, septal thickness 1.5 cm, posterior wall thickness 1.0 cm, systolic anterior motion (SAM) of the mitral valve with septal contact, and outflow tract narrowing. The diagnosis is: