A 34-year-old woman has noticed a bloody discharge from the nipple of her left breast for the past 3 days. On physical examination, the skin of the breasts appears normal, and no masses are palpable. There is no axillary lymphadenopathy. She has regular menstrual cycles and is using oral contraceptives. Excisional biopsy is most likely to show which of the following lesions in her left breast?