Final answer:
Oral mucosa that is cyanotic is the most critical assessment finding and requires immediate intervention as it indicates a severe oxygen deficit in the bloodstream.
Step-by-step explanation:
In observing a client's face, the assessment finding that requires the most immediate intervention by the nurse is the oral mucosa that is cyanotic. Cyanosis of the oral mucosa indicates a severe lack of oxygen in the bloodstream, which is a medical emergency and requires prompt intervention to address potential respiratory or circulatory failure. This condition is more critical than jaundice, which is a sign of liver dysfunction, or matted and crusted eyelids, which could indicate an infection such as blepharitis, or even a flushed and diaphoretic face, which may indicate fever or other non-emergent issues.