**Core Concept**
The diffusion capacity of the lung (DL) measures the ability of the lungs to transfer gases, specifically carbon monoxide (CO), from the inhaled air into the blood. It is an important indicator of lung function, particularly in assessing the integrity of the alveolar-capillary membrane and the presence of pulmonary diseases.

**Why the Correct Answer is Right**
The diffusion capacity of the lung (DL) is primarily dependent on the surface area available for gas exchange and the thickness of the alveolar-capillary membrane. Conditions that affect the alveolar-capillary membrane, such as interstitial lung diseases, can lead to a decrease in DL. However, conditions that primarily affect the airways, such as asthma or chronic bronchitis, may not significantly impact DL.

**Why Each Wrong Option is Incorrect**
**Option A:** Interstitial lung diseases, such as idiopathic pulmonary fibrosis, can lead to a decrease in DL due to the thickening of the alveolar-capillary membrane.
**Option B:** Pulmonary embolism can cause a decrease in DL due to the blockage of blood flow to the lungs, leading to impaired gas exchange.
**Option C:** Pulmonary edema can cause a decrease in DL due to the accumulation of fluid in the alveoli, which can impair gas exchange.

**Clinical Pearl / High-Yield Fact**
It's essential to note that DL is not affected by conditions that primarily affect the airways, such as asthma or chronic bronchitis. This is because DL measures the ability of the lungs to transfer gases, which is not directly affected by airway diseases.

**Correct Answer:** D.