Final answer:
The most reliable indicator of the existence and intensity of acute pain is the patient's subjective report. Nurses can use pain scales and patient descriptions to assess pain levels. Physiological measurements may also be used, but they may not always accurately represent pain intensity.
Step-by-step explanation:
The most reliable indicator of the existence and intensity of acute pain is the patient's subjective report. When a nurse is assessing a client's pain level, they will rely on the patient to describe and rate their pain. This can be done using pain scales such as the Wong-Baker Faces pain-rating scale, where the patient assigns a numerical value to their pain on a scale of 0-10.
Another method of assessing pain is through physiological measurements such as skin conductance fluctuations, which reflect sweating due to skin sympathetic nerve activity resulting from the stressor of pain. However, these physiological measures may not always accurately represent the intensity or subjective experience of pain.