This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Endocrine, Subject name: Pathology
A patient presented with neck swelling. Cytology showed showed parafollicular cells along with clusters of plasmacytoid and few spindle shaped cells. What investigation should be done to follow up the patient? 
Select and explain. 
A. Calcitonin, B. TSH level, C. Anti TPO antibody, D. TRH
A: Calcitonin
Answer: A: Calcitonin 
Explanation: Ans. (a) CalcitoninCytology showed showed parafollicular cells along with clusters of plasmacytoid and few spindle shaped cells are suggestive of medullary ca thyroid and so should be followed up by its tumor marker calcitonin.