Final answer:
The ductus arteriosus connects the pulmonary trunk and the aorta in the developing fetus, diverting blood away from the non-functioning fetal lungs.
Step-by-step explanation:
In the developing fetus, the structure that connects the pulmonary trunk and the aorta is known as the ductus arteriosus. It is a temporary vessel that allows most of the blood pumped from the right ventricle into the pulmonary trunk to be diverted into the aorta. This shunting is critical because the fetal lungs are not yet functioning for gas exchange, as the fetus receives oxygenated blood through the placenta. After birth, the ductus arteriosus closes as part of the transition to independent lung function.