A 22-year-old woman presents to the breast clinic with a 2-month history of a painless, mobile lump in the upper outer quadrant of her left breast. On examination, a 1.5 cm firm, well-circumscribed, highly mobile mass is palpated. Mammography is not performed due to her age. Ultrasound shows a hypoechoic, homogeneous, oval mass with sharp margins and no posterior acoustic shadowing. What is the most appropriate next step in management?