Oxygenated blood returns from pulmonary veins and enters
what part of the heart?

It enters the right atrium through the right ventricle.

Explanation: . The right ventricle pumps the blood to the lungs where it becomes oxygenated. The oxygenated blood is brought back to the heart by the pulmonary veins which enter the left atrium.

answer; genes dictate the production of specific enzymes, and affected individuals have genetic defects that cause them to lack certain enzymes;