**Core Concept**
Multiple myeloma is a type of cancer characterized by the proliferation of malignant plasma cells in the bone marrow. This leads to an imbalance in the production of various proteins and ions, resulting in several metabolic abnormalities.

**Why the Correct Answer is Right**
Hypercalcemia is a common metabolic abnormality seen in multiple myeloma due to the following reasons:
The excessive proliferation of plasma cells in the bone marrow leads to the release of osteoclast-activating factors, such as parathyroid hormone-related protein (PTHrP). This results in increased bone resorption and release of calcium ions into the bloodstream. Additionally, the production of monoclonal immunoglobulins by malignant plasma cells can lead to the suppression of osteoblastic activity, further contributing to hypercalcemia.

**Why Each Wrong Option is Incorrect**
**Option A:** Hyponatremia is not a characteristic metabolic abnormality in multiple myeloma, although it can be seen in various other conditions such as heart failure or liver disease.
**Option B:** Hypokalemia is not typically associated with multiple myeloma, although it can occur in patients with renal impairment or as a side effect of certain medications.
**Option D:** Hyperphosphatemia is not a direct consequence of multiple myeloma, although it can be seen in patients with renal impairment or as a side effect of certain medications.

**Clinical Pearl / High-Yield Fact**
Hypercalcemia in multiple myeloma can be a medical emergency, leading to symptoms such as confusion, weakness, and cardiac arrhythmias. It is essential to investigate and manage hypercalcemia promptly to prevent complications.

**✓ Correct Answer: C. Hypercalcemia**