Final answer:
The best principle to apply when managing pain in an 84-year-old woman with a humeral fracture is to monitor for a paradoxical increase in pain with opioid administration.
Step-by-step explanation:
When applying the nursing process in pain management for an 84-year-old woman with a humeral fracture, the best principle to apply is to monitor for a paradoxical increase in pain with opioid administration. As people age, they may experience altered responses to opioids, including an increased sensitivity to these medications. This can result in a paradoxical increase in pain rather than pain relief.