Final answer:
A Postnatally Patent Ductus Arteriosus (PDA) causes a Left-to-right shunt, allowing blood to flow from the higher pressure aorta to the lower pressure pulmonary trunk.
Step-by-step explanation:
A Postnatally Patent Ductus Arteriosus (PDA) causes a Left-to-right shunt, allowing blood to flow from the higher pressure aorta to the lower pressure pulmonary trunk. This additional flow of blood towards the lungs increases pulmonary pressure and makes respiration difficult. If left untreated, it can lead to congestive heart failure.