In parathyroid carcinoma
**Core Concept**
Parathyroid carcinoma is a rare malignancy of the parathyroid gland, accounting for approximately 1% of cases of primary hyperparathyroidism. The underlying pathophysiology involves uncontrolled cell growth and hormone production, leading to hypercalcemia.
**Why the Correct Answer is Right**
Parathyroid carcinoma typically presents with severe hypercalcemia due to excessive production of parathyroid hormone (PTH). The tumor cells exhibit autonomous secretion of PTH, which is not suppressed by normal feedback mechanisms, leading to an increase in serum calcium levels. This is in contrast to primary hyperparathyroidism due to adenoma or hyperplasia, where PTH production is still responsive to calcium levels.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because parathyroid adenoma, not carcinoma, is the most common cause of primary hyperparathyroidism.
**Option B:** This option is incorrect because hyperparathyroidism due to hyperplasia typically presents with mild to moderate hypercalcemia, not severe as seen in carcinoma.
**Option C:** This option is incorrect because familial hypocalciuric hypercalcemia is a genetic disorder characterized by mild hypercalcemia and is not associated with parathyroid carcinoma.
**Clinical Pearl / High-Yield Fact**
Parathyroid carcinoma should be suspected in patients with primary hyperparathyroidism who present with severe hypercalcemia, particularly if there is a palpable neck mass or significant weight loss.
**Correct Answer: D.**