A 30-year-old woman presents to the clinic with a painful swelling in her right breast for the past five days. She reports associated redness and warmth in the area, along with intermittent fevers and chills. She is currently breastfeeding her 3-month-old infant and states that the symptoms started after noticing a sore nipple. She denies any history of prior breast infections or trauma. The patient explains that she initially experienced mild tenderness in the right breast, which progressed to localized swelling and increased pain. She attempted warm compresses and frequent breastfeeding, but the symptoms worsened. She also noticed a decrease in milk output from the affected breast. Her medical history is unremarkable, and she takes no regular medications. She is a non-smoker. On examination, the patient appears uncomfortable but not acutely ill. The right breast shows an area of erythema, warmth, and induration in the upper outer quadrant. A fluctuant mass approximately 4 cm in diameter is palpable, with tenderness on palpation. There is no nipple discharge or overlying skin breakdown. Axillary lymph nodes are mildly enlarged and tender. The left breast is normal, with no signs of infection or tenderness. Ultrasound of the right breast reveals a well-defined, hypoechoic fluid collection consistent with an abscess. What is the most likely causative organism?
