<s>[INST] context: This patient's breast mass is characteristic of a fibroadenoma. Fibroadenomas are the second most common benign breast disorder, after fibrocystic changes. They are characterized by being firm, solid, nontender, and freely mobile. Fibroadenomas have an average size diameter of 2.5 cm and are well circumscribed. These lesions most commonly occur in adolescents and women in their twenties. Fibrocystic changes occur in about one- third to one-half of reproductive-age women and represent an exaggerated response of the breast tissue to hormones. Patients with fibrocystic changes complain of bilateral mastalgia and breast engorgement preceding menses. On physical exam, diffuse bilateral nodularity is typically encountered. Cystosarcoma phyllodes are rare fibroepithelial tumors that constitute 1% of breast malignancies. These rapidly growing tumors are the most frequent breast sarcoma and occur most frequently in women in the fifth decade of life. Trauma to the breast can result in fat necrosis. Women with fat necrosis commonly present to the physician with a firm, tender mass that is surrounded by ecchymosis. Occasional skin retraction can occur, making this lesion difficult to differentiate from cancer. It is unlikely that this patient who presents in her twenties has breast cancer. Fine-needle aspiration or excisional biopsy must be performed to rule out the rare chance of malignancy, but breast cancer is not the most likely diagnosis based on the patient's age and lack of any other breast changes consistent with carcinoma (such as a fixed mass, skin retraction, or lymphadenopathy). [/INST] question: A 28-year-old G0, LMP 1 week ago, presents to your gynecology clinic complaining of a mass in her left breast that she discovered on a routine breast self-exam in the shower. When you perform a breast exam on her, you palpate a 2-cm firm, nontender mass in the upper inner quadrant of the left breast that is well circumscribed and mobile. You do not detect any skin changes, nipple discharge, or lymphadenopathy. What is this patient's most likely diagnosis? | options: A: Fibrocystic breast change B: Fibroadenoma C: Breast carcinoma D: Fat necrosis | answer: B </s>