Which structure is involved in forming the trunk that divides into the aorta and pulmonary artery?

• A. Bulbus Cordis
• B. Truncus Arteriosus
• C. Primitive Atrium
• D. Endocardial Cushions

Answer: (B)

The key idea is how the outflow tract of the heart is organized during early development. In the embryo, the single outflow tract is called the truncus arteriosus, which will later be partitioned to form the aorta and the pulmonary trunk. The division happens via the aorticopulmonary septum, a spiral septum formed by neural crest cells migrating into this region (the outflow tract in concert with the nearby bulbus cordis). Because the trunk that ultimately divides into the aorta and pulmonary artery is the truncus arteriosus, this structure is the one involved in forming that trunk. The bulbus cordis contributes to the same general outflow region but the specific trunk that splits is the truncus arteriosus, and the division is accomplished by the aorticopulmonary septum formed within it. Endocardial cushions mainly handle AV septation and valve formation, not this outflow-trunk partition.