**Core Concept**
Humans possess four parathyroid glands, located in the neck region, which play a crucial role in regulating calcium and phosphate levels in the blood through the secretion of parathyroid hormone (PTH).

**Why the Correct Answer is Right**
The parathyroid glands are embedded in the posterior aspect of the thyroid gland and are responsible for maintaining calcium homeostasis in the body. Each gland produces PTH, which acts on the bones, kidneys, and intestines to increase calcium levels in the blood. The four parathyroid glands are usually divided into two pairs: the superior parathyroid glands, which are located near the posterior aspect of the thyroid gland, and the inferior parathyroid glands, which are located near the recurrent laryngeal nerve. The presence of four parathyroid glands allows for a redundant system, ensuring that calcium levels are maintained even if one gland is non-functional.

**Why Each Wrong Option is Incorrect**
**Option B:** This option is incorrect because the parathyroid glands are typically divided into two pairs, not three.
**Option C:** This option is incorrect because the parathyroid glands are usually found in pairs, not singularly.
**Option D:** This option is incorrect because the parathyroid glands are typically found in pairs, not a single gland.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that parathyroid gland dysfunction can lead to hyperparathyroidism, which is characterized by elevated levels of PTH and calcium in the blood. This can result in symptoms such as kidney stones, osteoporosis, and neuropsychiatric manifestations.

**✓ Correct Answer: A. 4 Humans have four parathyroid glands.**