This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Thyroid Gland, Subject name: Surgery
Which of the following is not true about medullary thyroid carcinoma? 
Select and explain. 
A. Are a type of neuroendocrine tumors, B. Associated with MEN 2A and 2B syndrome, C. Form about 25% of the thyroid malignancies, D. Tumor cells may secrete serotonin, ACTH and VIP
C: Form about 25% of the thyroid malignancies
Answer: C: Form about 25% of the thyroid malignancies 
Explanation: 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.