A 30-year-old woman sustained a traumatic blow to her right breast. Initially, there was a 3-cm contusion beneath the skin that resolved within 3 weeks, but she then felt a firm, painless lump that persisted below the site of the bruise 1 month later. What is the most likely diagnosis for this lump?

Fat necrosis is typically caused by trauma to the breast. The damaged, necrotic fat is phagocytosed by macrophages, which become lipid-laden. The lesion resolves as a collagenous scar within weeks to months. The firm scar can mammographically and grossly resemble a carcinoma. An abscess may form a palpable but painful mass lesion, and often from Staphylococcus aureus infection when localized. A fibroadenoma is a neoplasm, and tumors are not induced by trauma. Inflammatory carcinoma refers to dermal lymphatic invasion by an underlying breast carcinoma, giving a rough red-to-orange appearance to the skin. Sclerosing adenosis is a feature of fibrocystic changes, a common cause of nontraumatic breast lumps.