Truncus Arteriosus. A cyanotic congenital malformation in which the aorta and pulmonary artery fail to separate. This leads to one large vessel that receives blood from both the right and left ventricles. A large ventricular septal defect below the single valvular structure leads to a mixture of blood from both sides being taken into systemic circulation. The amounts of blood entering systemic vs. pulmonary circulation depend on the degree of resistance in the two circulations.