Final answer:
Nursing research studies are necessary to validate and improve nursing care specifically, complementing medical research done by organizations like the AMA, thus ensuring evidence-based best practices in the nursing profession.
Step-by-step explanation:
The necessity of a nursing research study despite existing research by the American Medical Association (AMA) is multifaceted. Nursing research is directed towards understanding and evidencing the best nursing care practices, which may be different from general medical practices that the AMA might research. Therefore, the correct response to the client's question would be:
D) This study is useful because it validates nursing care, not medical care.
Nursing research can complement and build upon medical research, but it has a distinct focus on the nursing profession and nursing care procedures. It ensures the evidence base for nursing is robust, applicable, and continuously updated to reflect the best practices in patient care. Moreover, such research can lead to improvements in patient outcomes by specifically targeting nursing interventions.