Truncus arteriosus is a diagnosis with anatomic components including:

A. VSD.

B. Abnormal origin of pulmonary arteries.

D. Single ventricular outflow valve.

 DISCUSSION: By definition, a VSD is always present immediately beneath the truncal valve. The pulmonary arteries arise abnormally from the single trunk, which is due to failed partitioning of the embryonic conus during the first few weeks of fetal development. In the classification of Collett and Edwards, Type I truncus arteriosus has a single arterial trunk giving rise to an aorta and a main pulmonary artery; in Type II the right pulmonary arteries arise immediately adjacent to one another from the dorsal wall of the truncus; in Type III the right and left pulmonary arteries originate from either side of the truncus; and in Type IV the proximal pulmonary arteries are absent and pulmonary blood flow is by way of major aortopulmonary atresia and is no longer considered truncus arteriosus. Subaortic stenosis cannot occur in this anomaly. The single ventricular outflow valve is the truncal valve. It may contain from two to six cusps, but most often there are three and, next most often, four.