A 2-day-old newborn male is admitted to the pediatric intensive care unit with cyanosis and tachypnea. Cardiac ultrasound and MRI examinations reveal totally anomalous pulmonary connections. Which of the following embryologic events is responsible for this malformation?

The right horn of the sinus venosus has two divisions: One develops into the sinus venarum, the smooth interior aspect of the right atrial wall; the other half develops into the pulmonary veins. Abnormal septation of the sinus venosus can lead to inappropriate pulmonary connections. Abnormal development of the left sinus horn would present with abnormalities in the coronary sinus, whereas incorrect development of the septum secundum can result in an atrial septal defect but would not be involved with anomalous pulmonary veins. The left sinual horn develops into the coronary sinus, and the right sinual horn is incorporated into the right atrial wall.