This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Thyroid Tumor, Subject name: Pathology
A 42-year-old woman presents with a diffuse swelling in front of the neck. Thyroid function tests are within normal limits. A fine-needle biopsy reveals malignant cells. The tumor is excised and examined. The tumour cells are positive for calcitonin on immunohistochemistry. What is the appropriate pathologic diagnosis? 
Select and explain. 
A. Anaplastic carcinoma, B. Follicular carcinoma, C. Medullary carcinoma, D. Papillary carcinoma
C: Medullary carcinoma
Answer: C: Medullary carcinoma 
Explanation: Medullary carcinoma thyroid microscopic features: Composed of polygonal to spindle-shaped cells, which may form nests, trabeculae, and even follicles. Small, more anaplastic cells are present in some tumors and may be the predominant cell type. Amyloid deposits derived from calcitonin polypeptides are present in the stroma in many cases. Calcitonin is readily demonstrable within the cytoplasm of the tumor cells as well as in the stromal amyloid by immunohistochemical methods. Electron microscopy reveals variable numbers of membrane-bound electron-dense granules within the cytoplasm of the neoplastic cell.