**Core Concept**
Thyroid malignancies are a diverse group of cancers arising from the thyroid gland, with varying incidence rates. The most common thyroid cancers are papillary, follicular, and medullary thyroid carcinomas, while less common types include anaplastic and undifferentiated thyroid cancers.

**Why the Correct Answer is Right**
The least common thyroid malignancy is generally considered to be lymphoma of the thyroid. This type of cancer accounts for a small percentage of all thyroid cancers, typically affecting older adults. Lymphoma of the thyroid often presents with symptoms such as neck mass, hoarseness, and difficulty swallowing, and may be associated with underlying thyroiditis or Hashimoto's thyroiditis.

**Why Each Wrong Option is Incorrect**
**Option A:** Anaplastic thyroid carcinoma is a rare and aggressive form of thyroid cancer, but it is more common than lymphoma of the thyroid.
**Option B:** Medullary thyroid carcinoma is a neuroendocrine tumor that arises from the parafollicular cells of the thyroid gland, but it is more common than lymphoma of the thyroid.
**Option C:** Undifferentiated thyroid carcinoma is a rare and aggressive form of thyroid cancer, but it is more common than lymphoma of the thyroid.

**Clinical Pearl / High-Yield Fact**
Lymphoma of the thyroid is often associated with underlying autoimmune thyroiditis, such as Hashimoto's thyroiditis, and may present with symptoms similar to those of thyroiditis, including neck mass and pain.

**Correct Answer: D. Lymphoma of the thyroid.**