**Core Concept**
Medullary carcinoma of the thyroid (MTC) is a type of neuroendocrine tumor that originates from the parafollicular cells (also known as C-cells) of the thyroid gland. The treatment of choice for MTC involves a multidisciplinary approach, including surgery, medical therapy, and radiotherapy.

**Why the Correct Answer is Right**
The treatment of choice for medullary carcinoma of the thyroid is surgical resection, specifically a total thyroidectomy. This is because MTC is a localized disease at the time of diagnosis, and surgical removal of the tumor is the most effective way to achieve cure. The surgery aims to remove the entire thyroid gland, along with the lymph nodes in the neck that may contain cancer cells. The primary goal of surgery is to achieve complete resection of the tumor, thereby reducing the risk of recurrence and improving survival.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because while external beam radiation therapy may be used as an adjunct to surgery in certain cases, it is not the primary treatment for MTC.
**Option B:** This option is incorrect because chemotherapy is not the standard treatment for MTC. Although some patients with advanced disease may be treated with chemotherapy, it is not the first-line treatment.
**Option C:** This option is incorrect because targeted therapy with tyrosine kinase inhibitors (TKIs) may be used in the treatment of advanced MTC, but it is not the primary treatment for localized disease.

**Clinical Pearl / High-Yield Fact**
It's essential to note that medullary carcinoma of the thyroid can be associated with multiple endocrine neoplasia (MEN) syndromes, particularly MEN 2A and MEN 2B. Therefore, patients with a family history of MTC or other endocrine tumors should be screened for these syndromes.

**Correct Answer:** D.