What is the likely cause?
These features suggest mitral regurgitation. The main differential is of mitral stenosis. Presence of significant left ventricular enlargement is a feature of mitral regurgitation (volume overload) but is not seen in mitral stenosis.
What are some of the causes of chronic mitral regurgitation?
Myxomatous degeneration, previous infective / inflammatory illness (e.g. rheumatic heart disease, infective endocarditis), connective tissue diseases, congenital valvular malformation, dilated cardiomyopathy.
The rough outline of the left atrium (yellow) can be inferred by the presence of a double border on the right (green arrow), splaying of the carina (blue) and prominence of the left atrial appendage (red arrow).