You are answering a USMLE-style medical question. Question: A 41-year-old woman presents with shortness of breath that is worse when she lies on her left side. About 10 days ago, she had an episode of unexplained loss of consciousness. Past medical history is negative and family history is irrelevant. Clinical examination shows a diastolic murmur, which is prominent when she lies on her left side. Jugular venous distention is present, and chest examination reveals fine crackles that do not clear with coughing. Chest X-ray shows pulmonary congestion, and 2-dimensional echocardiogram shows a mass in the left atrium attached to the atrial septum. Which of the following is the most likely diagnosis? Options: A) Rheumatic fever B) Innocent murmur C) Non-bacterial thrombotic endocarditis D) Atrial myxoma Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.