A 58-year-old woman presents with an irregular nodularity that has developed in her right breast over the past 3 months. Mammography demonstrates irregular densities in both breasts. A needle biopsy of one breast lesion is shown. An excisional biopsy of the contralateral breast shows similar histology. Which of the following is the most likely pathologic diagnosis?
Colloid carcinoma Lobular carcinoma in situ Malignant phyllodes tumor Medullary carcinoma