<prompt> All are true about medullary thyroid carcinoma except:Involves the parafollicular cellRadiosensitiveAmyloid stroma is presentElevated level of calcitonin  <response>: Answer- B. RadiosensitiveThese are tumours of the parafollicular C cells derived from the neural crest.There is a characteristic amyloid stroma.High levels of serum calcitonin and carcinoembryonic antigen are produced medullary tumours.Calcitonin levels fall efter resection and rise again with recurrence making it a valuable tumour marker in the follow up of patients with this disease.Diarrhoea due to 5-hydroxytrptamine or prostaglandins.Tumours are not TSH dependent and do not take up radioactive iodine. 