**Core Concept**
The endocrine system is a network of glands that produce hormones to regulate various bodily functions. Cancer of the endocrine system, also known as endocrine tumors or endocrine cancers, arises from the abnormal growth of cells in these glands. The most common type of endocrine cancer is thyroid cancer, but among the endocrine glands, the most common malignancy is actually of the adrenal glands.

**Why the Correct Answer is Right**
The adrenal glands are located on top of the kidneys and produce hormones like adrenaline (epinephrine) and cortisol, which are essential for stress response, metabolism, and blood pressure regulation. Adrenal gland cancers, also known as adrenocortical cancers, are rare but can be aggressive and malignant. These cancers can produce excessive amounts of hormones, leading to symptoms like weight gain, high blood pressure, and excess hair growth.

**Why Each Wrong Option is Incorrect**
* **Option A:** Thyroid cancer is indeed a common endocrine malignancy, but it is not the most common one among the endocrine glands. Thyroid cancer originates from the thyroid gland, which is located in the neck and produces hormones that regulate metabolism.
* **Option B:** Pancreatic cancer is not an endocrine cancer, but a type of exocrine cancer that arises from the pancreas, which is an organ that produces digestive enzymes and hormones like insulin and glucagon.
* **Option C:** Pituitary gland cancer is a rare type of endocrine cancer that arises from the pituitary gland, which is located at the base of the brain and produces hormones that regulate growth, metabolism, and reproductive functions.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that adrenal gland cancer is often associated with excessive hormone production, which can lead to symptoms like weight gain, high blood pressure, and excess hair growth.

**Correct Answer:** D.