Final answer:
The key sign of onset of acute respiratory distress syndrome (ARDS) is hypoxemia, which is a lower-than-normal level of oxygen in the blood.
Step-by-step explanation:
The key sign of onset of acute respiratory distress syndrome (ARDS) is hypoxemia, which is a lower-than-normal level of oxygen in the blood. ARDS is a condition that affects the lungs and can be caused by various factors such as trauma, infection, or inhaling harmful substances. In ARDS, the lungs become inflamed and filled with fluid, making it difficult for oxygen to enter the bloodstream.