**Core Concept**
Hypercalcemia crisis, also known as hypercalcemic crisis, is a severe and life-threatening condition characterized by extremely elevated serum calcium levels, often exceeding 14 mg/dL. This condition can lead to cardiac arrest, respiratory failure, and renal failure if not promptly treated.

**Why the Correct Answer is Right**
The most common cause of hypercalcemia crisis is malignancy, particularly squamous cell carcinoma of the head and neck. This is due to the production of parathyroid hormone-related protein (PTHrP) by tumor cells, which mimics the action of parathyroid hormone (PTH) and leads to excessive calcium release from bones. PTHrP stimulates osteoclasts to break down bone, resulting in high levels of calcium in the blood.

**Why Each Wrong Option is Incorrect**
**Option A:** While primary hyperparathyroidism can cause hypercalcemia, it is not the most common cause of hypercalcemia crisis.
**Option B:** Vitamin D intoxication can cause hypercalcemia, but it is relatively rare and not typically associated with a hypercalcemic crisis.
**Option C:** Familial hypocalciuric hypercalcemia (FHH) is a genetic disorder that can cause hypercalcemia, but it is not a common cause of hypercalcemia crisis.

**Clinical Pearl / High-Yield Fact**
Remember that hypercalcemia crisis is a medical emergency that requires prompt treatment, including hydration, bisphosphonates, and calcitonin. Early recognition and treatment can prevent serious complications and improve patient outcomes.

**Correct Answer: C. Malignancy, particularly squamous cell carcinoma of the head and neck.**