A 22-year-old woman presents to the outpatient clinic with a painless lump in her left breast that she noticed six months ago. The lump has remained the same size, and she denies symptoms such as nipple discharge, skin changes, or breast pain. She has no significant medical history, is not on any medications, and has no family history of breast cancer. On physical examination, the lump is located in the upper outer quadrant of the left breast. It is firm, smooth, well-circumscribed, non-tender, and mobile. There is no axillary lymphadenopathy or overlying skin changes. An ultrasound of the breast reveals a well-defined, homogeneous, hypoechoic lesion with smooth margins, measuring 2.5 cm in diameter, with no calcifications. Which of the following is the most likely diagnosis?
