This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Surgery
Screening method of medullary carcinoma thyroid is: 
Select and explain. 
A. Serum calcitonin, B. Serum calcium, C. Serum alkaline phosphatase, D. Serum acid phosphatase
A: Serum calcitonin
Answer: A: Serum calcitonin 
Explanation: Medullary Carcinoma Thyroid (MTC):

Neuroendocrine carcinoma arising from parafollicular ‘C’ cells of thyroid.
Parafollicular ‘C’ cells are derived from the ultimobranchial bodies and secrete calcitonin.
‘C’ Cells are concentrated superolaterally in thyroid lobes, from where MTC usually develops.
Most MTCs (75-80%) arise sporadically.
Spread is both lymphatic and hematogenous.
MC site of metastasis: Liver