Phyllodes tumor

A 55-year-old woman comes to the clinic due to a left breast mass. She first noticed it a few months ago, and since then, it has progressively grown in size. Medical history is significant for diabetes mellitus type 2, for which she takes metformin. She does not smoke, drink alcohol or use illicit drugs. Vitals are within normal limits. On physical examination, a smooth, well-defined, firm and mobile mass is noted in the left breast. There is no associated pain, and the rest of the physical examination is within normal limits. Biopsy is performed, and the results are shown below:  


 Reproduced from: Wikimedia Commons

This patient’s lesion most likely arises from which of the following areas?