A. Instructing the client in the use of the PCA pump directly addresses the severe pain by allowing the client to self-administer analgesia as needed, which provides immediate relief and is crucial for effective pain management post-surgery. B. Assisting the client in changing positions may offer temporary relief but does not address the underlying pain, which should be managed primarily through medication. C. Initiating CPM is not an appropriate first step for managing severe pain. CPM is used for improving joint mobility and should only be considered after effective pain management is established. D. Applying ice might provide some temporary relief, but it is not a substitute for effective pain control through medication. Addressing the pain with PCA is the priority.