A 40-year-old patient is diagnosed with a localized 1 cm infiltrating ductal cancer after a needle core biopsy of the lesion. She is clinical node negative; a lumpectomy and sentinel lymph node biopsy are performed. The patient develops an anaphylactic response during the case. Which of the following substances was the likely causative agent?

Both methylene blue or isosulfan (lymphazurin) blue dye can be used for sentinel lymph node identification and have been associated with some allergic reactions. Isosulfan blue has been associated with rare anaphylactic reactions in <1% of patients. Methylene blue can cause skin necrosis if injected too superficially. Fluorenscein and indigo carmine are not given in these surgeries. 99 Tc is given for the lymphoscintigraphy and gamma probe isolation of the sentinel node and has no known anaphylactic reactions.