From the question on page 580

The MRI shows cor triatriatum (fig 1). This is a rare congenital abnormality accounting for 0.1% of all congenital cardiac malformations.1 It is caused by a failure of common pulmonary vein resorption during cardiogenesis. This leads to persistence of a diaphragm-like membrane dividing the left atrium. Communication between the chambers is via one or more orifices. The size of this communication determines the …