Which of the following is a cause of hypoxemia?

A right-to-left shunt is a significant cause of hypoxemia because it allows deoxygenated blood to bypass the lungs and enter the systemic circulation directly. In a healthy respiratory system, blood flows through the lungs where it gets oxygenated. However, in conditions where there is a right-to-left shunt, such as congenital heart defects (e.g., tetralogy of Fallot) or pulmonary arteriovenous malformations, deoxygenated blood mixes with oxygenated blood. As a result, the oxygen content of the arterial blood is reduced, leading to hypoxemia, which is defined as decreased oxygen levels in the blood.

Hyperventilation typically results in increased oxygen levels in the blood because it increases the amount of air entering the alveoli, allowing for greater gas exchange. Increased inspired O2 tension refers to conditions where the concentration of oxygen in the inhaled air is elevated, thereby increasing oxygen uptake. Enhanced diffusion capacity is associated with a more effective transfer of oxygen from the alveoli to the bloodstream, which would also lead to improved rather than decreased oxygenation.

Therefore, the only mechanism among the options listed that directly contributes to hypoxemia by reducing oxygen saturation in systemic circulation is a right-to-left shunt.