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

Which type of lung disease results in a reduced TLC largely due to limited inspiration?

1. Obstructive Lung Capacity

2. Right-to-Left Shunt

3. Physiological Shunt

4. Restrictive Lung Disease

The correct answer is: Restrictive Lung Disease

Restrictive lung disease is characterized by a reduction in total lung capacity (TLC) primarily due to limited expansion of the lungs during inspiration. This limitation can arise from various factors, including intrinsic lung pathologies (such as pulmonary fibrosis) that cause stiffening of the lung tissue or extrinsic factors, like obesity or chest wall deformities, that hinder lung expansion. In restrictive lung disease, the lung volumes and capacities are diminished, but the airflow rates are generally normal or even increased since the air passage is not obstructed; rather, it is the lung's ability to contain air that is impaired. This contrasts with obstructive lung diseases like asthma or chronic obstructive pulmonary disease (COPD), where TLC may be normal or increased, but the ability to expel air is limited, leading to hyperinflation. Conditions such as right-to-left shunt and physiological shunt refer to issues related to blood flow and gas exchange rather than direct restrictions in lung volume; thus, they do not appropriately fit the context of decreased TLC due to limited inspiration.