Which condition could result in a decreased diffusing capacity in a patient?

A decreased diffusing capacity, often measured by the diffusing capacity of the lungs for carbon monoxide (DLCO), indicates impaired gas exchange in the lungs. Interstitial lung disease is characterized by inflammation and fibrosis of the lung interstitium, which can severely affect the alveolar architecture and capillary bed. This damage reduces the surface area available for gas exchange, leading to a decreased diffusing capacity.

In contrast, while conditions like bronchiectasis, COPD, and pneumonia can impact overall lung function, they do not primarily affect the alveolar-capillary membrane in the way that interstitial lung disease does. COPD may cause airflow obstruction and lead to air trapping, but its effects on gas exchange differ from the specific destruction of membrane integrity seen in interstitial lung diseases. Pneumonia may lead to some impaired gas exchange due to consolidation, but it is often reversible. Bronchiectasis can cause airway damage and chronic infection, which might decrease diffusing capacity to some extent but not as significantly or directly as interstitial lung disease does. Therefore, interstitial lung disease is the condition most clearly associated with decreased diffusing capacity.