The advanced practice registered nurse (APRN) is assessing a 55 year-old cisgender female who presents to the primary clinic with complaints of a non-tender mass in her left breast. The patient's last mammogram was two years ago, and was normal. During the clinical breast exam the APRN notes a single, firm, Irregularly shaped mass with poorly delineated borders in the left breast. What is the most likely cause of the breast mass?