Q. A 55-year-old male patient presents to the clinic with a lump in his neck that has been slowly growing over the past year. On physical examination, a firm, fixed, non-tender mass is palpated in the lower pole of the right thyroid lobe. The patient has no family history of thyroid cancer or any other endocrine disorders. Laboratory tests reveal an elevated serum calcitonin level. A fine needle aspiration biopsy of the thyroid mass is performed and reveals sheets of polygonal cells in an amyloid stroma on histological examination. The cells stain positively with Congo red. What is the most likely diagnosis for this patient?