A 52-year-old man from Delhi presents with progressive dyspnea on exertion and orthopnea for 3 months. On examination, he has a loud S1, an opening snap, and a low-pitched diastolic murmur best heard at the apex in the left lateral decubitus position. Chest X-ray shows pulmonary congestion and straightening of the left heart border. Which investigation is most appropriate to assess the severity of mitral stenosis and guide management?