**Core Concept**
Adult respiratory distress syndrome (ARDS) is a life-threatening condition characterized by acute onset of hypoxemic respiratory failure, typically following a severe insult to the lung or systemic circulation. The pathophysiology of ARDS involves diffuse alveolar damage, leading to impaired gas exchange and increased pulmonary vascular permeability.

**Why the Correct Answer is Right**
The characteristic feature of ARDS is the development of non-cardiogenic pulmonary edema, which is not responsive to fluid challenge. This occurs due to increased permeability of the alveolar-capillary membrane, allowing fluid, proteins, and inflammatory cells to leak into the alveoli. The resulting edema leads to hypoxemia and respiratory failure. The release of inflammatory mediators, such as cytokines and chemokines, plays a crucial role in the initiation and progression of ARDS.

**Why Each Wrong Option is Incorrect**
* **Option A:** While hypoxemia is a hallmark of ARDS, it is not the characteristic feature of the syndrome. Hypoxemia is a consequence of the underlying pathophysiology.
* **Option B:** Pulmonary embolism can cause acute respiratory distress, but it is not the characteristic feature of ARDS. Pulmonary embolism typically presents with a more abrupt onset and a higher likelihood of cardiac complications.
* **Option C:** Pneumonia can lead to respiratory failure, but it is not the characteristic feature of ARDS. Pneumonia often presents with fever, cough, and sputum production, whereas ARDS is typically characterized by a more subtle presentation.

**Clinical Pearl / High-Yield Fact**
Remember that ARDS is characterized by a non-cardiogenic pulmonary edema, which is not responsive to fluid challenge. This is a key distinction from cardiogenic pulmonary edema, which is often responsive to fluid management.

**Correct Answer:** C. Non-cardiogenic pulmonary edema.