**Core Concept**
Medullary carcinoma of the thyroid (MTC) is a rare, malignant tumor originating from the parafollicular cells (also known as C cells) of the thyroid gland. The treatment of MTC involves a multidisciplinary approach, including surgery, medical therapy, and targeted therapy.

**Why the Correct Answer is Right**
The treatment of MTC involves surgical resection of the primary tumor, which is the mainstay of therapy. The goal of surgery is to achieve a complete resection of the tumor, which improves survival and reduces recurrence rates. In addition, surgery also helps to reduce symptoms and improve quality of life. Targeted therapy with tyrosine kinase inhibitors (TKIs), such as vandetanib and cabozantinib, has also been shown to be effective in treating MTC, particularly in patients with advanced disease.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because chemotherapy is not a standard treatment for MTC. While chemotherapy may be used in some cases, it is not a first-line treatment for this disease.
**Option B:** This option is incorrect because radiotherapy is not typically used as a primary treatment for MTC. While radiation therapy may be used in some cases, such as to control symptoms or prevent recurrence, it is not a standard treatment for this disease.
**Option C:** This option is incorrect because hormone replacement therapy (HRT) is not a treatment for MTC. HRT may be used to manage symptoms related to hypothyroidism or hyperthyroidism, but it is not a treatment for the cancer itself.

**Clinical Pearl / High-Yield Fact**
It's essential to note that MTC can be associated with multiple endocrine neoplasia type 2 (MEN2) syndromes, which are inherited conditions characterized by the presence of MTC, pheochromocytoma, and hyperparathyroidism. Genetic testing for RET mutations can help identify patients with these syndromes.

**Correct Answer: D. Surgical resection of the primary tumor, with or without targeted therapy.