A 32-year-old woman from rural India presents with progressive dyspnea on exertion and orthopnea for 6 months. On examination, she has a low-volume pulse, a tapping apical impulse, and a loud S1 with an opening snap. A diastolic murmur is heard at the apex. 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?