You are answering a USMLE-style medical question. Question: A 45-year-old woman comes to the emergency department because of intermittent chest pain and palpitations. She also complains of dyspnea on exertion and has had a 4.5-kg (10-lb) weight loss in the past 2 months despite normal appetite. She has no history of serious medical illness. Cardiac examination shows an irregularly irregular rhythm and a grade 3/6, low pitched, rumbling, mid-diastolic murmur heard best over the apex. Diffuse wheezing and bilateral rales are present in the lung bases. A transthoracic echocardiogram shows a left-ventricular ejection fraction of 40% and a mass in the left atrium. A biopsy of the mass is likely to show which of the following? Options: A) Nonencapsulated groups of well-differentiated skeletal muscle cells B) Mixture of cells from different germinal layers C) Encapsulated cluster of mature fat cells D) Nests of atypical melanocytes Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.