**Core Concept**
Hypercalcemia and hypophosphatemia can occur together in several conditions, often due to increased bone resorption or renal phosphate wasting. This question requires the student to identify the underlying cause.

**Why the Correct Answer is Right**
The correct answer involves a condition characterized by increased parathyroid hormone (PTH) levels, which stimulates bone resorption, leading to increased calcium and decreased phosphate levels. Elevated PTH also increases renal phosphate excretion.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not directly relate to the combination of hypercalcemia and hypophosphatemia.
* **Option B:** This option is incorrect because it refers to a different electrolyte imbalance, not directly related to the question.
* **Option C:** This option is incorrect because it is a condition characterized by decreased PTH, which would not lead to increased calcium and decreased phosphate levels.

**Clinical Pearl / High-Yield Fact**
When encountering a patient with hypercalcemia and hypophosphatemia, consider evaluating PTH levels and renal function to identify the underlying cause.

**Correct Answer: C. Primary hyperparathyroidism.**