**Core Concept**
Medullary carcinoma of the thyroid (MTC) is a type of neuroendocrine tumor originating from the parafollicular cells (also known as C cells) of the thyroid gland. MTC is characterized by its ability to secrete calcitonin, a hormone involved in calcium homeostasis. The tumor's behavior and spread are influenced by its neuroendocrine nature.

**Why the Correct Answer is Right**
The most common site of metastasis for MTC is the lymph nodes, particularly those in the neck and mediastinum. This is due to the tumor's tendency to spread through the lymphatic system, which is a hallmark of neuroendocrine tumors. The lymph nodes act as a filter for cancer cells, allowing them to accumulate and eventually form metastatic deposits. In the case of MTC, the lymph nodes in the jugular chain and mediastinum are commonly involved.

**Why Each Wrong Option is Incorrect**
**Option A:** Bone is not a common site of metastasis for MTC. While MTC can spread to the bones, it is not the most frequent site of metastasis.
**Option B:** Liver is not typically the first site of metastasis for MTC. Although the liver can be involved in advanced disease, it is not the most common location for initial metastasis.
**Option C:** Lung is not a primary site of metastasis for MTC. While MTC can spread to the lungs, it is not the most common location for metastasis, especially in comparison to lymph nodes.

**Clinical Pearl / High-Yield Fact**
MTC is often associated with multiple endocrine neoplasia (MEN) syndromes, particularly MEN 2A and MEN 2B. These syndromes are characterized by the presence of other endocrine tumors, such as pheochromocytoma and hyperparathyroidism.

**Correct Answer:** C. Lymph nodes.