**Core Concept**
Hyperparathyroidism is a condition characterized by excessive secretion of parathyroid hormone (PTH) from one or more parathyroid glands. This leads to elevated calcium levels in the blood, causing a range of symptoms and complications. The most common cause of hyperparathyroidism is a benign tumor, or adenoma, on one of the parathyroid glands.

**Why the Correct Answer is Right**
A single adenoma is the most common cause of primary hyperparathyroidism, accounting for approximately 80-85% of cases. This is because a solitary adenoma can autonomously produce excess PTH, leading to hypercalcemia and hyperparathyroidism. The adenoma can be small, and its presence may not be detectable through routine imaging or examination. The increased PTH secretion stimulates the parathyroid hormone-related protein (PTHrP) pathway, leading to increased calcium levels and decreased phosphate levels in the blood.

**Why Each Wrong Option is Incorrect**
**Option B:** Multiple adenomas are rarely the cause of hyperparathyroidism, as the simultaneous presence of multiple adenomas on different glands is uncommon. However, when it does occur, it is often associated with familial multiple endocrine neoplasia (MEN) syndromes.

**Option C:** Single gland hyperplasia refers to the enlargement of one parathyroid gland due to an increase in the number of parathyroid cells. While this can cause hyperparathyroidism, it is less common than a single adenoma.

**Option D:** Multiple gland hyperplasia, also known as diffuse parathyroid hyperplasia, is a condition where all four parathyroid glands are enlarged due to an increase in the number of parathyroid cells. This is the second most common cause of hyperparathyroidism after a single adenoma, but it is still less common.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that a single adenoma is the most common cause of primary hyperparathyroidism, and it can often be treated with surgical removal of the affected gland. This can lead to a significant improvement in symptoms and a reduction in the risk of complications.

**✓ Correct Answer: A. Single adenoma**