A 27-year-old woman in the third trimester of her third pregnancy discovers a lump in her left breast. On physical examination, a 2-cm, discrete, freely movable mass beneath the nipple is palpable. After the birth of a term infant, the mass appears to decrease in size. The infant is breastfed without difficulty. What is the most likely diagnosis?
Fibroadenoma Intraductal papilloma Lobular carcinoma in situ Medullary carcinoma