What is the most likely diagnosis on this transthoracic echocardiogram? A patient in their 70s, with a remote history of dilated cardiomyopathy (which was lost to follow-up), permanent atrial fibrillation, and hypertension, presented with dyspnea and signs suggestive of decompensated heart failure. A transthoracic echocardiogram revealed severe biventricular dilatation with severe functional mitral and tricuspid regurgitation. The forward left ventricular ejection fraction was severely reduced and measured 25%. Strikingly, there was evidence of marked biatrial dilatation, with a measured indexed left atrial volume of 438.3 mL/m2 (Figure).