Final answer:
The nurse should use the Wong-Baker Faces scale to assess the mentally challenged client's pain.
Step-by-step explanation:
The nurse should use the Wong-Baker Faces scale to assess the mentally challenged client's pain. This scale uses a series of faces with different expressions to represent different levels of pain. The client can choose the face that best represents their pain level. This scale is widely used in various populations, including children and individuals with cognitive impairments.
The Faces scale is particularly suitable for individuals who may have difficulty understanding or communicating using numeric or word scales. It provides a visual representation of pain intensity and allows the nurse to quickly assess the client's pain level.
Using the Faces scale, the nurse can ask the client to point to the face that best matches their pain. The scale ranges from 0 to 10, with 0 being no pain and 10 being the worst pain imaginable.