**Core Concept**
Parathyroid adenoma and hyperplasia are both causes of primary hyperparathyroidism, but they differ in their pathophysiology and clinical presentation. Parathyroid adenoma is a solitary tumor of a parathyroid gland, whereas hyperplasia involves the enlargement of multiple parathyroid glands.

**Why the Correct Answer is Right**
To differentiate between parathyroid adenoma and hyperplasia, the following features can be considered:
- **Option B:** The number of affected parathyroid glands. In adenoma, only one gland is involved, whereas in hyperplasia, multiple glands are affected.
- **Option C:** The histological appearance. Adenoma typically shows a well-defined boundary with surrounding tissue, whereas hyperplasia shows diffuse enlargement of the gland with a more irregular boundary.
- **Option D:** The serum calcium levels. While both conditions can cause elevated serum calcium levels, adenoma tends to cause more severe hypercalcemia due to the increased production of parathyroid hormone (PTH) by a single gland.

**Why Each Wrong Option is Incorrect**
**Option A:** The size of the parathyroid gland is not a reliable feature to differentiate between adenoma and hyperplasia, as both conditions can cause significant enlargement of the gland.

**Clinical Pearl / High-Yield Fact**
Remember that parathyroid adenoma is more likely to cause severe hypercalcemia and is often associated with a single gland that is palpable on physical examination, whereas hyperplasia is often associated with multiple glands that are not palpable.

**Correct Answer:** B. The number of affected parathyroid glands.