**Core Concept**
The lung diffusing capacity (DL) measures the ability of the lungs to transfer gases from the airspaces to the bloodstream. It is primarily assessed by measuring the transfer of carbon monoxide (CO) or nitric oxide (NO) across the alveolar-capillary membrane. A low DL indicates impaired gas exchange, which can be caused by various conditions affecting the lung parenchyma, airspaces, or the alveolar-capillary membrane.

**Why the Correct Answer is Right**
A low lung diffusing capacity is typically caused by conditions that destroy or damage the alveolar-capillary membrane, such as interstitial lung diseases (e.g., idiopathic pulmonary fibrosis, sarcoidosis), pulmonary vascular diseases (e.g., pulmonary embolism, pulmonary hypertension), or conditions that lead to alveolar destruction (e.g., emphysema, cystic fibrosis). Option C, pulmonary embolism, would indeed cause a low DL due to the blockage of capillaries in the lungs, leading to impaired gas exchange.

**Why Each Wrong Option is Incorrect**

* **Option A:** Chronic obstructive pulmonary disease (COPD) would likely cause a low DL due to the destruction of alveolar walls in emphysema, leading to impaired gas exchange.
* **Option B:** Pulmonary edema would cause a low DL due to the accumulation of fluid in the alveolar spaces, which would impair gas exchange.

**Clinical Pearl / High-Yield Fact**
Remember that a normal DL is crucial for assessing the severity of pulmonary diseases, especially in patients with chronic respiratory conditions. A low DL can also be an indicator of other systemic conditions, such as rheumatoid arthritis or systemic lupus erythematosus, which can cause interstitial lung disease.

**Correct Answer:** C. Pulmonary embolism.