Final answer:
Patent ductus arteriosus (PDA) causes left to right shunting of blood from the aorta to the main pulmonary artery, leading to a step-up in oxygen saturation from the right ventricle to the pulmonary artery.
Step-by-step explanation:
The medical condition causing left to right shunting of blood from the aorta to the main pulmonary artery is patent ductus arteriosus (PDA). This condition would lead to a step-up in oxygen saturation from the right ventricle to the pulmonary artery.
In fetal development, the ductus arteriosus is a normal blood vessel that connects the pulmonary artery to the aorta, bypassing the lungs because the fetus gets oxygen through the placenta, not the lungs. After birth, this vessel is supposed to close as part of the normal transition to breathing air. When it remains open, or patent, it's called patent ductus arteriosus. This patent connection allows oxygen-rich (oxygenated) blood from the higher-pressure aorta to mix with oxygen-poor (deoxygenated) blood in the pulmonary artery. Consequently, more blood goes to the lungs, and an increase in oxygen saturation can be observed between the right ventricle and the pulmonary artery. As the blood is shunted, it can lead to the progressive symptoms and complications associated with PDA.