A 35-year-old woman presents with a 1.5 cm firm, mobile breast mass in the left breast that has been stable for 2 years. Ultrasound shows a well-defined, hypoechoic oval lesion with sharp margins and posterior acoustic enhancement, consistent with fibroadenoma. The patient is asymptomatic and requests confirmation before deciding on management. Which investigation would provide the most specific tissue diagnosis with the least morbidity?