A 35-yrs-old woman presents with a 2.2 -cm mass in her left breast. The mass is excised, and histologic sections reveal a tumor composed of a mixture of ducts and cells. The epithelial cells within the ducts are not atypical in appearance. There is a marked increase in the stromal cellularity, but the stromal cells are not atypical in appearance and mitoses are not found. Which of the following is the most likely diagnosis?