As with any reference interview, see if you can get as much background as possible. Ask to see the assignment. For many of the health science programs, and nursing in particular, the professor is looking for a single study -- a specific questionnaire, clinical trial or observational study that was done on a group (patients, nurses, etc.). What the professor usually doesn't want is a larger systematic review or meta-analysis on a topic (those are covered in Evidence-Based Medicine on the left).
One approach to try is limiting by Publication Type on the initial search. It's not perfect but it helps. Publication types that usually get to research articles are Clinical Trial, Randomized Controlled Trial, and just plain Research. (EBSCO provides descriptions of these publication types). See the video below for a sample search in CINAHL.