A 28-year-old G0, LMP 1 week ago, presents to your gynecology clinic complaining of a mass in her left breast that she discovered on a routine breast self-exam in the shower. When you perform a breast exam on her, you palpate a 2-cm firm, nontender mass in the upper inner quadrant of the left breast that is well circumscribed and mobile. You do not detect any skin changes, nipple discharge, or lymphadenopathy. What is this patient's most likely diagnosis?