Which of the following is not a cause of increased shunting?

• A. Pulmonary emboli
• B. Atelectasis
• C. Pulmonary edema
• D. Pneumonia

Answer: (A)

Shunt occurs when blood flows through parts of the lung that are not adequately ventilated, so it passes through without picking up oxygen. Atelectasis collapses alveoli, pneumonia fills air spaces with fluid or exudate, and pulmonary edema floods the alveoli—each scenario reduces ventilation relative to perfusion and increases the amount of blood that returns poorly oxygenated, i.e., increases shunt.

A pulmonary embolism, by contrast, blocks blood flow to portions of the lung, creating dead space where air is present but not perfused. This reduces effective gas exchange through loss of perfused units rather than adding poorly ventilated units. So it does not increase shunting; it increases dead space ventilation.