Most common cause of hypercalcemic crisis is:
**Core Concept**
Hypercalcemic crisis is a life-threatening condition characterized by severely elevated serum calcium levels, often exceeding 14 mg/dL. It can lead to cardiac arrest, respiratory failure, and renal failure if not promptly treated. The most common causes of hypercalcemic crisis involve excessive bone resorption, usually due to malignancy or excessive vitamin D levels.
**Why the Correct Answer is Right**
The correct answer is **Malignancy**. Malignant cells can produce various factors that stimulate osteoclasts to resorb bone, leading to hypercalcemia. This is often seen in multiple myeloma, lung cancer, and breast cancer. The parathyroid hormone-related protein (PTHrP) is a key mediator of this process, which mimics the action of parathyroid hormone (PTH) and increases calcium levels in the blood. The mechanism involves the activation of protein kinase C (PKC) and the subsequent increase in osteoclast activity.
**Why Each Wrong Option is Incorrect**
**Option A:** Hyperparathyroidism is a common cause of hypercalcemia, but it usually does not progress to a crisis. Hyperparathyroidism involves the overproduction of PTH, which stimulates bone resorption, but it is a more chronic condition.
**Option B:** Familial hypocalciuric hypercalcemia (FHH) is a rare genetic disorder that causes hypercalcemia, but it is not typically associated with a crisis. FHH involves a mutation in the calcium-sensing receptor (CaSR), which leads to increased PTH production and hypercalcemia.
**Option C:** Vitamin D toxicity can cause hypercalcemia, but it is less common than malignancy as a cause of hypercalcemic crisis. Vitamin D toxicity involves excessive levels of 1,25-dihydroxyvitamin D, which increases calcium absorption from the gut and bone resorption.
**Clinical Pearl / High-Yield Fact**
Malignancy-induced hypercalcemia is often associated with a high level of PTHrP, which can be detected in the serum. This is a key distinguishing feature from primary hyperparathyroidism, where PTH levels are typically elevated.
**Correct Answer: C. Malignancy. Malignancy-induced hypercalcemia.**