A 28-year-old woman from rural India presents with dyspnea on exertion and palpitations for 6 months. On examination, she has a loud S1, opening snap, and a mid-diastolic murmur best heard at the apex in the left lateral decubitus position. Chest X-ray shows straightening of the left heart border and pulmonary congestion. Which investigation is most appropriate to confirm the diagnosis and assess severity of mitral stenosis?