A 3-day-old newborn was born with ectopia cordis. Despite the efforts of doctors at the pediatric intensive care unit the infant died from cardiac failure and hypoxemia. Which of the following embryologic events is most likely responsible for the development of such conditions?

Ectopia cordis is a condition in which the heart is located abnormally outside the thoracic cavity, commonly resulting from a failure of fusion of the lateral folds in forming the thoracic wall. This is incompatible with life because of the occurrence of infection, cardiac failure, or hypoxemia. Faulty development of the sinus venosus is related to atrial septal defects that result from deficient absorption of the sinus venosus into the right atrium and/or unusual development of the septum secundum.