This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: General pathology, Subject name: Pathology
A 75-year-old lady is diagnosed of having a thyroid malignancy. Her serum calcium is low. What is the most likely explanation? 
Select and explain. 
A. She has developed metastasis of the parathyroid, B. She has follicular carcinoma, C. She has papillary carcinoma, D. She has medullary carcinoma
D: She has medullary carcinoma
Answer: D: She has medullary carcinoma 
Explanation: It originates from parafollicular cells that produce calcitonin. 3rd most common thyroid cancer. Calcitonin-highly sensitive marker. For screening. Differential diagnosisi. Prognosis. Assessment of treatment response.