Final answer:
The nurse at the outpatient clinic should prioritize the phone message from the client with a C6 spinal cord injury reporting a headache (option D) , as it may indicate autonomic dysreflexia, which is a medical emergency.
Step-by-step explanation:
In prioritizing the phone messages that the nurse in an outpatient clinic received, it is crucial to identify which situation poses the most immediate health risk to the patient. The nurse should return the call from the client diagnosed with a C6 spinal cord injury and reporting a headache first. This symptom may indicate the potentially life-threatening condition known as autonomic dysreflexia, which requires immediate medical attention.
The other calls, while important, do not present the same level of urgency. Watery diarrhea in an older adult client undergoing bowel prep is expected; breast engorgement in a new mother is uncomfortable but not an emergency; and nausea post-cataract extraction, while needing follow-up, does not generally pose immediate danger as the headache in a patient with spinal cord injury does.