congenital malformation in which the aorta arises from the morphologic right ventricle and the pulmonary artery from the morphologic left ventricle resulting in two separate and parallel circulations. The condition is lethal unless some communication exists between the systemic and pulmonic circulation after birth; otherwise, unoxygenated venous blood inappropriately enters the systemic circulation, and oxygenated pulmonary venous blood is inappropriately directed to the pulmonary circulation. The life-sustaining communication may be an intraatrial passage or a patent ductus arteriosus. SYN: transposition of arterial stems.