Review Question - QID 214518

A 45-year-old nulligravid woman presents to the office with intermittent copper-colored discharge from her left breast for the past month. The patient performs monthly self-breast examinations and has not felt any lumps during this time. Her history is significant for schizophrenia which has been well controlled with risperidone and type 2 diabetes for which she takes metformin. She does not use tobacco, alcohol, or illicit drugs. Family history is significant for breast cancer in her mother at age 60. The patient’s temperature is 99.3°F (37.4°C), blood pressure is 120/86 mmHg, pulse is 80/min, and respirations are 15/min. On physical exam, 1 ml of bloody discharge is expressed from her left nipple. No mass, tenderness, breast discoloration, or lymphadenopathy is present on palpation. Which of the following is the most likely cause of this patient’s findings?