Which of the following actions should a nurse take when assessing a client’s response to pain medication during evaluation?

Comparing pre and post-treatment pain levels is essential in evaluating a client's response to pain medication. This approach allows the nurse to quantify the effectiveness of the medication and understand how well it is managing the client's pain. By measuring pain levels before and after administering the medication, the nurse can better understand the treatment's efficacy and make informed decisions about continuing, adjusting, or changing the medication as needed for optimal patient care.

Documenting only the physical appearance does not provide a comprehensive view of how the patient is experiencing pain relief. Ignoring client feedback is counterproductive, as the patient’s subjective experience of pain is crucial for effective pain management. Assessing a different patient’s response would not address the individual needs and response of the patient in question, which is the focus needed in this scenario. Thus, comparing pre and post-treatment pain levels is the most effective action in this context.