(A) H&E low power: Medullary thyroid carcinoma with the typical solid nested architecture.
(B) H&E high power: Medullary thyroid carcinoma with the typical solid nested architecture. The tumor cells have neuroendocrine type nuclei and can be plasmacytoid (center of the nest).
(C) Calcitonin immunostain: In almost all cases, the tumor is diffusely positive for calcitonin.