A 25-year-old patient during selfexamination detected a tumor in the upper external quadrant of the right mammary gland. On palpation: painless, dense, mobile growth 2 cm in diameter is detected in the mammary gland; no changes in the peripheral lymph nodes are observed. On mammary glands US: in the upper external quadrant of the right mammary gland there is a space-occupying lesion of increased echogenicity 21x18 mm in size. The most likely diagnosis is: