A 42-year-old woman presents with a swollen, painful, erythematous left breast which does not respond to a 10-day course of oxacillin. Ultrasound reveals no abscess. The next step in management should be:
Begin a 10-day course of vancomycin Workup the patient for an immunosuppressive disease Incise and drain the area Biopsy the skin and parenchyma of the breast