A 42-year-old woman has noted increasing fullness in her neck for the past 7 months. On physical examination, her thyroid gland is diffusely and asymmetrically enlarged and nodular. There is no lymphadenopathy. She undergoes thyroidectomy. Gross examination of the thyroid shows a multicentric thyroid neoplasm; microscopically, the neoplasm is composed of polygonal-to spindle-shaped cells forming nests and trabeculae. There is a prominent, pink hyaline stroma that stains positively with Congo red. Electron microscopy shows varying numbers of intracytoplasmic, membrane-bound, electron-dense granules. Immunohistochemical staining for which of the following antigens is most useful for the diagnosis of this neoplasm?