Pulmonary Stenosis Blocks Right Ventricular Outflow

Steps:

• Blood flows from the right ventricle through the pulmonary valve into the pulmonary artery.
• Pulmonary stenosis narrows the pulmonary valve, obstructing this outflow.
• This directly reduces blood volume entering the pulmonary artery.
• The pulmonary artery carries blood to the lungs for oxygenation, so lung perfusion drops first.

Why C is correct:

• The pulmonary artery directly supplies deoxygenated blood to the lungs; stenosis impedes flow here per basic cardiac anatomy.

Why the others are wrong:

• A: Brain receives oxygenated blood via systemic arteries from the left ventricle, affected only after lung oxygenation fails.
• B: Left ventricle fills from pulmonary veins after lung circulation, so reduction is secondary.
• D: Left atrium receives blood from pulmonary veins post-lung oxygenation, making it indirect.

Final answer: C