**Core Concept**
Acute Respiratory Distress Syndrome (ARDS) is a life-threatening condition characterized by rapid onset of inflammation in the lungs, leading to impaired gas exchange and hypoxemia. It is a clinical syndrome that presents with hypoxemia, non-cardiogenic pulmonary edema, and sometimes hypercapnia.

**Why the Correct Answer is Right**
ARDS is characterized by a "stiff lung" or increased lung elastance, making it difficult to expand the lungs during mechanical ventilation. This is due to the inflammation and injury to the type II pneumocytes, leading to the release of surfactant and the collapse of alveoli. However, the term "stiff lung" is more commonly associated with chronic obstructive pulmonary disease (COPD) and interstitial lung disease (ILD). In ARDS, the lung is actually more prone to over-inflation and barotrauma due to the loss of surfactant and the increased lung elastance.

**Why Each Wrong Option is Incorrect**
**Option A:** Hypoxemia is a hallmark of ARDS, resulting from the impaired gas exchange in the lungs.
**Option B:** Hypercapnia can occur in ARDS, particularly in patients who are not mechanically ventilated or have a high respiratory rate.
**Option C:** Pulmonary edema is a key feature of ARDS, resulting from the increased permeability of the alveolar-capillary membrane.

**Clinical Pearl / High-Yield Fact**
In ARDS, the use of positive end-expiratory pressure (PEEP) is crucial to prevent alveolar collapse and improve gas exchange. However, excessive PEEP can lead to over-inflation of the lungs and barotrauma.

**✓ Correct Answer:** D. Stiff lung