<s>[INST] context: The most distinctive clue is the presence of amyloid, which specifically suggests medullary thyroid carcinoma. This carcinoma is a tumor of the neuroendocrine parafollicular cells of the thyroid, rather than the follicular lining epithelium. These cells produce calcitonin, the precursor protein of which can precipitate, forming bands and nodules of amyloid that appear histologically identical to other forms of amyloid. The other thing to remember about medullary carcinoma of the thyroid is that it can be a component of multiple endocrine neoplasia (MEN) syndromes type IIa (parathyroid disease, pheochromocytomas, medullary carcinoma) and type IIb (parathyroid disease, pheochromocytomas, medullary carcinoma, mucocutaneous ganglioneuromas, Marfanoid habitus).Because follicular thyroid carcinoma closely resembles normal thyroid tissue, it usually cannot be reliably diagnosed based on fine needle aspiration alone. Hashimoto's disease would show lymphocytes, plasma cells, and macrophages on aspiration.Papillary thyroid carcinoma can be diagnosed by aspiration if papillary clusters are seen. [/INST] question: During an employment health check at Infosys, Bangalore, a 45-year-old man, Narayan, is noted to have a 3 cm palpable nodule in one lobe of an otherwise normal sized thyroid gland. Needle aspiration of the nodule demonstrates polygonal tumor cells and amyloid, but only very scanty colloid and normal follicular cells. Which of the following is the most likely diagnosis? | options: A: Follicular thyroid carcinoma B: Hashimoto's disease C: Medullary thyroid carcinoma D: Papillary thyroid carcinoma | answer: C </s>