**Core Concept**
Parathyroid carcinoma is a rare malignancy of the parathyroid glands, often associated with primary hyperparathyroidism. The diagnosis of parathyroid carcinoma can be challenging due to its similarity in presentation to benign parathyroid adenomas. A key diagnostic feature of parathyroid carcinoma is the presence of metastasis, which distinguishes it from its benign counterparts.

**Why the Correct Answer is Right**
The correct answer is metastasis because parathyroid carcinoma spreads to distant sites, such as the lungs, liver, and bones, through the bloodstream and lymphatic system. This is in contrast to benign parathyroid adenomas, which do not metastasize. The presence of metastasis is a critical diagnostic feature, as it indicates a more aggressive tumor behavior and necessitates a more thorough evaluation and treatment plan. The metastatic spread of parathyroid carcinoma is thought to occur through the parathyroid hormone-related protein (PTHrP) pathway, which is overexpressed in malignant parathyroid tissue.

**Why Each Wrong Option is Incorrect**
**Option A:** Cytology, while useful in diagnosing parathyroid lesions, is not a definitive diagnostic feature of parathyroid carcinoma. Cytological examination may reveal atypical cells, but it cannot reliably distinguish between benign and malignant lesions.

**Option C:** Clinical features, such as hypercalcemia and hyperparathyroidism, are nonspecific and can be seen in both benign and malignant parathyroid lesions. While these features may raise suspicion for parathyroid carcinoma, they are not diagnostic on their own.

**Clinical Pearl / High-Yield Fact**
Remember that the presence of metastasis is a key diagnostic feature of parathyroid carcinoma. In the absence of metastasis, a diagnosis of parathyroid carcinoma cannot be made, even in the presence of atypical cytology or clinical features.

**Correct Answer Line**
✓ Correct Answer: B. Metastasis