63 year old woman with shortness of breath and weakness for many years, has been told she has a 'hole in her heart'

This chest radiograph shows a very enlarged heart (blue outlines indicate the normal size of the heart), and markedly increased fullness of all pulmonary vessels, which can be seen in large left-to-right shunt lesions. The CT series shows the very large heart as well as a more subtle abnormality in the atria. Try to find the right and left atria and see what you think might be the cause of her symptoms.