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:

The patient should have a biopsy of the skin and breast tissues. The most likely diagnosis is inflammatory breast cancer. The skin should be included in order to assess for invasion of the dermal lymphatics. Dermal lymphatic invasion is not mandatory for the diagnosis of inflammatory breast cancer. Incision and drainage is not indicated as no abscess was seen on ultrasound. This is not likely to be caused by methicillin-resistant S. aureus; therefore a course of vancomycin would not be indicated. The other answers are not appropriate even if the problem was an infection.