Why coarctation of aorta in infantile, cause passes of the blood from pulmonary artery to the aorta through pda?

Other: Blood flows from higher pressure to lower pressure. Aorta higher, pa lower usually. Coarctation distal to PDA cause higher aortic pressure spikes with obstruction and higher pa flow from aorta to pa through the pda-patent ductus arteriosus.

Coarctation : The narrowed aorta causes a lower pressure beyond the narrowing which allows flow from the PDA into the aorta.

Ductal dependence: Coarctation is usually associated with a small ao arch and/or aortic stenosis. If this is the case, the only way blood can get to the body (or systemic circulation) is thru the pda. The PDA is often associated with the coarctation and the narrowing may worsen as the PDA closes. This is why the coarctation may be "ductal dependent".

Plumbing!: Blood must get to the body, else the fetus would not survive. The same is true once the baby is born. In utero, blood flows from pulm artery (pa) to aorta (ao) normally. Once born, the blood continues to flow this way so that blood can get to the body. When the PDA closes, the child becomes very sick.