A 58-year-old man from Delhi presents with progressive dyspnea on exertion, orthopnea, and paroxysmal nocturnal dyspnea for 3 months. On examination, he has an opening snap followed by a low-pitched diastolic murmur at the apex. Chest X-ray shows pulmonary edema and straightening of the left heart border. Which investigation is most appropriate to confirm the diagnosis and assess severity?