Final answer:
The American Nurses Association (ANA) is the organization that represents nurses and addresses healthcare issues at the national level.
Step-by-step explanation:
The organization that represents nurses and addresses healthcare issues on a national level is the American Nurses Association (ANA). This professional association is dedicated to advancing the nursing profession by fostering high standards of nursing practice, promoting a safe and ethical work environment, bolstering the health and wellness of nurses, and advocating on healthcare issues that affect nurses and the public. Other organizations mentioned, such as the American Medical Association (AMA), National League of Nursing (NLN), and Sigma Theta Tau (STT), also contribute to the medical and nursing fields but with different primary foci.