Hypoxemia from cardiac shunting causes polycythemia

Steps:

• Congenital heart defect with hole (e.g., VSD) allows deoxygenated blood from right side to mix with oxygenated blood on left side.
• Mixing reduces oxygen saturation in blood delivered to body tissues.
• Chronic low oxygen (hypoxemia) stimulates kidneys to release erythropoietin.
• Erythropoietin boosts bone marrow production of red blood cells to carry more oxygen.

Why D is correct:

• Lower oxygen concentration (hypoxemia) triggers erythropoietin-mediated increase in red blood cells, per physiological response to hypoxia.

Why the others are wrong:

• A: Faster pumping increases circulation but does not directly cause red cell increase; hypoxia does.
• B: Lower pressure from shunting does not require more blood volume or red cells.
• C: Hemoglobin affinity decreases in low oxygen, not increases; red cell count rises instead.

Final answer: D