Q: What structure guards the orifice through which the blood in the mammalian heart pumped by the right ventricle passes out?  

Answer: D

The pulmonary valve is located between the right ventricle and the pulmonary artery, and it prevents the backflow of blood from the pulmonary artery into the right ventricle. When the right ventricle contracts, the pulmonary valve opens, allowing blood to be pumped out of the heart and into the lungs. The pulmonary valve then closes when the right ventricle relaxes to prevent the backflow of blood. Therefore, option 4 is the correct answer.

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