Which of the following conditions is known to decrease pulmonary vascular resistance (PVR)?

The correct choice is related to the physiological response of the pulmonary vasculature to changes in blood volume. Hypovolemia, or decreased blood volume, can lead to a decrease in pulmonary vascular resistance (PVR) due to a reduction in blood flow through the pulmonary arteries. When there is less blood available, the vascular system is able to dilate more easily in response to local conditions, which can lower resistance.

In general, a reduction in PVR is beneficial for improving blood flow to the lungs and enhancing oxygenation. Other factors, such as hypoxemia, hypertension, and airway obstruction, tend to increase resistance rather than decrease it. Specifically, hypoxemia can cause vasoconstriction in pulmonary vessels, hypertension increases pressure and resistance within those vessels, and airway obstruction can lead to changes in ventilation and perfusion ratios that also elevate PVR. Thus, the physiological effects of hypovolemia distinctly result in decreased pulmonary vascular resistance, making it the correct answer in this context.