<prompt> Which of the following is not true about medullary thyroid carcinoma?Are a type of neuroendocrine tumorsAssociated with MEN 2A and 2B syndromeForm about 25 of the thyroid malignanciesTumor cells may secrete serotonin, ACTH and VIP  <response>: Ans. is c i.e., Form about 25 of the thyroid malignancies Medullary carcinoma of thyroid Medullary carcinomas of the thyroid are neuroendocrine neoplasms derived from the parafollicular cells, or C cells, of the thyroid, and account for approximately 5 of thyroid neoplasms. Medullary carcinomas, similar to normal C cells, secrete calcitoniny the measurement of which plays an important role in the diagnosis and postoperative follow-up of patients. In some instances the tumor cells elaborate other polypeptide hormones, such as serotonin, ACTH, and vasoactive intestinal peptide VIP. About 70 of tumors arise sporadically. The remainder occurs in the setting of MEN syndrome 2A or 2B or as familial tumors without an associated MEN syndrome. Activating point mutations in the RET proto-oncogene play an important role in the development of both familial and sporadic medullary carcinomas. 