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

What is a typical cause of an Elevated Alveolar-Arterial Gradient related to Diffusion defects?

1. Asbestosis

2. Emphysema

3. Pulmonary fibrosis

4. Ureteric Bud issue

The correct answer is: Pulmonary fibrosis

An elevated alveolar-arterial (A-a) gradient related to diffusion defects typically indicates a problem with the transfer of oxygen from the alveoli into the blood due to impaired gas exchange. In the context of diffusion defects, pulmonary fibrosis is a classic example. Pulmonary fibrosis results in scarring and thickening of the alveolar walls, making it more difficult for oxygen to diffuse from the alveoli into the pulmonary capillary blood. This leads to a decrease in the amount of oxygen that enters the bloodstream, causing a significant increase in the A-a gradient, as the arterial oxygen levels become markedly lower than what would be expected based on the alveolar oxygen levels. Conditions like asbestosis and emphysema affect gas exchange but through different mechanisms. Asbestosis causes lung scarring but can also lead to restrictive lung disease, while emphysema primarily causes a decrease in surface area due to destruction of alveolar walls but does not typically lead to a significant diffusion defect. The ureteric bud issue mentioned is unrelated to pulmonary function and does not pertain to diffusion defects in the lungs. Thus, pulmonary fibrosis stands out as the primary cause of an elevated A-a gradient specifically due to diffusion defects.