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

What are the primary pathophysiologic mechanisms that reduce diffusing capacity in restrictive lung diseases?

1. Increased surfactant production

2. Alveolar destruction in emphysema

3. Atelectasis in asthma

4. Thickening of the pulmonary membrane

The correct answer is: Thickening of the pulmonary membrane

In restrictive lung diseases, the primary pathophysiologic mechanisms that reduce diffusing capacity are primarily related to the thickening of the pulmonary membrane. This thickening occurs due to various factors, such as fibrosis or inflammatory processes, which can lead to an increased distance between the alveoli and the pulmonary capillaries. As a result, gas exchange becomes less efficient, resulting in reduced diffusing capacity. In restrictive diseases, the lungs’ ability to expand is compromised, which also affects ventilation and oxygenation. The increased thickness of the alveolar-capillary membrane hinders the diffusion of gases (oxygen and carbon dioxide) across it, contributing significantly to the reduced diffusing capacity observed in these conditions. This contrasts with other options, where increased surfactant production does not lead to reduced diffusing capacity; instead, surfactant is crucial for maintaining alveolar stability and preventing collapse. Alveolar destruction is more characteristic of obstructive lung diseases like emphysema rather than restrictive lung diseases. Similarly, atelectasis is typically a reversible condition rather than a chronic pathophysiologic mechanism that would lead to reduced capacity in the context of restrictive lung disease.