**Core Concept**
The diffusion capacity of the lung (DLCO) is a measure of the lung's ability to transfer gas from the alveoli to the blood. It is primarily affected by diseases that damage the alveolar-capillary membrane or reduce the surface area available for gas exchange.

**Why the Correct Answer is Right**
DLCO is decreased in conditions that damage the alveolar-capillary membrane, such as interstitial lung diseases (ILDs), pulmonary fibrosis, and pulmonary edema. These conditions lead to thickening of the alveolar-capillary membrane, reducing the diffusion capacity. Conditions that do not damage the alveolar-capillary membrane, but rather affect the lung's ability to ventilate or perfuse, would not decrease DLCO.

**Why Each Wrong Option is Incorrect**
* **Option A:** Interstitial lung disease (ILD) would decrease DLCO due to thickening of the alveolar-capillary membrane.
* **Option B:** Pulmonary fibrosis would also decrease DLCO due to scarring and thickening of the alveolar-capillary membrane.
* **Option C:** Pulmonary edema would decrease DLCO due to fluid accumulation in the alveolar space, increasing the distance for gas exchange.

**Clinical Pearl / High-Yield Fact**
It's essential to note that DLCO is not affected by conditions that primarily affect lung volume, such as chronic obstructive pulmonary disease (COPD) or asthma, as long as lung volume is normal.

**Correct Answer: D.**