## Core Concept
Pleomorphic adenoma, also known as benign mixed tumor, is the most common type of salivary gland tumor, primarily affecting the parotid gland. It is characterized by its diverse cellular components, including both epithelial and mesenchymal elements. The tumor's behavior and treatment are influenced by its unique histological features.

## Why the Correct Answer is Right
The correct answer, , states that pleomorphic adenoma has a high rate of malignant transformation if left untreated. This is a well-documented characteristic of pleomorphic adenoma. The tumor has the potential to undergo malignant transformation, with an estimated risk that increases over time if the tumor is not surgically removed. This risk underscores the importance of surgical intervention.

## Why Each Wrong Option is Incorrect
* **Option A:** This option is incorrect because pleomorphic adenomas are indeed known for their diverse histological appearance, featuring a mix of epithelial and mesenchymal components.
* **Option B:** This option is incorrect as pleomorphic adenomas most commonly occur in the parotid gland, making them the most common salivary gland tumor.
* **Option D:** This option is incorrect because while recurrence can happen, especially if the tumor is not completely excised, stating it as a characteristic "except" does not directly address a commonly emphasized feature like malignant transformation.

## Clinical Pearl / High-Yield Fact
A key clinical pearl is that pleomorphic adenomas have a significant risk of malignant transformation if left untreated for a long period. The risk is estimated to be around 2-5% over 5-10 years. Early surgical removal is recommended to prevent this transformation.

## Correct Answer: D.