Neoplasia USMLE Step 1 Practice Question

A 32-year-old woman presents to her primary care physician after discovering a painless, mobile mass in her left breast during self-examination 2 months ago. She reports the lump has not changed in size. She is otherwise healthy, takes oral contraceptives, and denies nipple discharge, fever, or constitutional symptoms. Vital signs are normal. Physical examination confirms a well-defined, rubbery, movable mass measuring approximately 2.5 cm in the upper outer quadrant. Mammography shows a homogeneous, circumscribed lesion without calcifications or skin thickening. Core needle biopsy demonstrates benign glandular tissue with proliferating epithelial and myoepithelial cells arranged in interlacing fascicles, with preservation of the basement membrane and no invasion into surrounding adipose tissue. Which of the following is the most likely diagnosis?