Fibrocystic breast changes

A 40-year-old woman comes to the primary care clinic because she realized that a firm, round 2 cm mass in her right breast has not gone away after her menstrual period. The firm lump has remained for the last 6 weeks. She has had chronic tenderness in both breasts and she notes that multiple lumps in her breasts would come and go at different times in her menstrual cycle. Which of the following diagnostic modalities would confirm this patient's diagnosis?