Phyllodes tumor

A 45-year-old woman comes to the office because she is concerned she may have breast cancer. She says that while she was doing her monthly breast examination, she noticed a firm non-tender mass in her left breast. Her past medical history is significant only for hypertension, for which she takes hydralazine. She drinks about a glass of wine or two a week and does not smoke or use illicit drugs. Her family history is negative for cancer. Physical examination shows a mobile, multinodular tumor in the left lower quadrant of the breast that measures about 5 cm in diameter. The overlying skin appears stretched and non-erythematous. She has no lymphadenopathy. Which of the following is the most likely diagnosis?