Which of the following causes of hypoxia is characterized by a decreased arterial $\mathrm{PO}_{2}$ and an increased A-a gradient?

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[IV A 4; IV D; Table 4.4; Table 4.5]. Hypoxia is defined as decreased $\mathrm{O}_{2}$ delivery to the tissues. It occurs as a result of decreased blood flow or decreased $\mathrm{O}_{2}$ content of the blood. Decreased $\mathrm{O}_{2}$ content of the blood is caused by decreased hemoglobin concentration (anemia), decreased $\mathrm{O}_{2}$-binding capacity of hemoglobin (carbon monoxide poisoning), or decreased arterial $\mathrm{Po}_{2}$ (hypoxemia). Hypoventilation, right-to-left cardiac shunt, and ascent to high altitude all cause hypoxia by decreasing arterial $\mathrm{PO}_{2}$. Of these, only right-to-left cardiac shunt is associated with an increased A-a gradient, reflecting a lack of $\mathrm{O}_{2}$ equilibration between alveolar gas and systemic arterial blood. In right-to-left shunt, a portion of the right heart output, or pulmonary blood flow, is not oxygenated in the lungs and thereby "dilutes" the $\mathrm{PO}_{2}$ of the normally oxygenated blood. With hypoventilation and ascent to high altitude, both alveolar and arterial $\mathrm{PO}_{2}$ are decreased, but the A-a gradient is normal.

Which of the Following Causes of Hypoxia Is Characterized by a Decreased