Final answer:
The 2 cm left breast mass in the 27-year-old woman is most consistent with trauma, based on the biopsy results showing fat necrosis. Breast cancer, infection, and autoimmune disease are not likely causes.
Step-by-step explanation:
Based on the biopsy results showing fat necrosis, the most consistent etiology for the 2 cm left breast mass in the 27-year-old woman in the civil lawsuit would be Trauma.
Fat necrosis is a condition that occurs when there is trauma or injury to fatty tissue, leading to the death of fat cells. In this case, the bruise on her upper arm could have resulted in trauma to the breast tissue, causing the formation of the breast mass.
Breast cancer, infection, and autoimmune disease are not the likely causes in this scenario, as they would present with different histological findings.