Which of the following is a hallmark symptom of acute respiratory distress syndrome?

A. Refractory hypoxemia: This is the hallmark of ARDS. It means hypoxemia (low oxygen levels in the blood) that does not improve despite increasing oxygen therapy. It results from severe ventilation-perfusion mismatch and alveolar damage. B. Pink frothy sputum: This is more characteristic of pulmonary edema due to left-sided heart failure, not specifically ARDS. C. Hypoglycemia: Hypoglycemia is not a typical symptom of ARDS. It might occur due to other metabolic issues or sepsis, but it is not diagnostic. D. Hypercapnia: While CO₂ retention can occur in late stages, ARDS is primarily marked by oxygenation failure, not ventilatory failure initially.