A 45-year-old Indian woman presents with progressive dyspnea and orthopnea. On examination, she has an opening snap and a diastolic murmur at the apex. A posteroanterior chest X-ray is obtained. The structure marked **B** (left heart border) shows straightening of the left heart border with a double density sign on the right side of the mediastinum and splayed carina. Which of the following cardiac conditions best explains these radiological findings?