It's important to cite sources you used in your research for several reasons:
APA stands for the American Psychological Association and is the format designed for use within the field of psychology. However, other disciplines use APA as well, so always use the format your professor chooses.
Citing Medicine: the NLM style guide for authors, editors, and publishers
The official style manual for the National Library of Medicine (NLM), and suggested when citing documents from the associated databases MedLine and PubMed. The online edition supercedes the print edition as the most up to date.