A 27-year-old breastfeeding woman presents with swelling, redness, and tenderness in her left breast. What is the most likely diagnosis?

In the case of a 27-year-old breastfeeding woman presenting with swelling, redness, and tenderness in her left breast, the most logical diagnosis is mastitis. This condition often occurs in breastfeeding women due to the bacteria entering the breast tissue, typically through cracked nipples or milk stasis. The symptoms presented, including swelling, redness, and tenderness, are classic signs of mastitis, which may also be accompanied by flu-like symptoms such as fever.

While a galactocele (a cyst formed by a blocked milk duct) can occur in breastfeeding women, it typically presents with a palpable mass rather than the acute inflammatory symptoms seen in this scenario. The presentation of localized symptoms along with the context of recent breastfeeding strongly suggests an inflammatory process rather than a non-inflammatory one like a galactocele.

For the other options, fibroadenoma is a benign breast tumor mostly found in younger women but typically does not present with the acute symptoms of inflammation. An abscess can occur as a complication of mastitis but is a more advanced stage and is characterized by a localized collection of pus, which can be a consequence of untreated mastitis. However, given the initial presentation, mastitis is the most likely early diagnosis.