A 30-year-old female patient complains of milk discharge from the mammary glands, 5-month absence of menstruation. She had one physiological labour four years ago. Objectively: mammary glands are normally developed. Bimanual examination reveals that the uterus is decreased in size, the ovaries are of normal size. MRI-scan shows no cerebral pathologies. Concentration of thyroid-stimulating hormone is normal. The serum prolactin level is increased. What is the most likely diagnosis?