Review Question - QID 108607

A 42-year-old G3P3003 presents for a 6-week postpartum obstetric visit and complains of left breast redness, swelling, and pain. The patient has been breastfeeding since her recent spontaneous vaginal delivery, and after her symptoms began 4 weeks ago, she was treated with oral dicloxacillin without resolution of symptoms. She denies fevers or chills and has no past medical history or family history. Prior to becoming pregnant, the patient smoked one pack per day; she now smokes cigarettes only when stressed. Her infant is healthy and developing well. Temperature is 98.9°F (37.2°C), blood pressure is 114/78 mmHg, pulse is 86/min, and respirations are 14/min. On exam, the right breast appears normal while the left breast is shown in Figure A. There is no discrete palpable mass or axillary lymphadenopathy on either side, and no lymphadenopathy is present in the cervical or inguinal chains either. Which of the following is the most likely etiology of this patient’s presentation?