USMLE (United States Medical Licensing Examination) Step 1 Practice Exam

What condition is associated with an Elevated Alveolar-Arterial Gradient due to V/Q Mismatch?

1. Airway obstruction

2. Interstitial lung disease

3. Pulmonary embolism

4. Renal artery stenosis

The correct answer is: Pulmonary embolism

An elevated alveolar-arterial (A-a) gradient is commonly seen in conditions where there is a mismatch between ventilation (V) and perfusion (Q) in the lungs. This mismatch leads to impaired gas exchange, as some areas of the lung may be well ventilated but poorly perfused, or vice versa. Pulmonary embolism is a classic example of a condition that causes elevated A-a gradient due to V/Q mismatch. When a blood clot occludes a pulmonary artery, it prevents blood flow to a portion of the lung while still allowing ventilation to that area. This results in high ventilation compared to blood flow in the affected lung regions, leading to inadequate oxygenation of the blood, thus causing an increased A-a gradient. Other conditions like airway obstruction and interstitial lung disease can also alter V/Q ratios and affect gas exchange, but they typically present differently and may not always lead to the same pattern of A-a gradient elevation observed in pulmonary embolism. Renal artery stenosis is unrelated to respiratory physiology and does not influence the A-a gradient.