**Question:** The commonest tumour of parotid is

A. Warthin's tumour
B. Salivary gland carcinoma
C. Pleomorphic adenoma
D. Mucocele

**Correct Answer:** C. Pleomorphic adenoma

**Core Concept:**
The parotid gland is the largest salivary gland located in the face, primarily responsible for producing saliva and maintaining oral hydration. It is composed of two lobes, superficial and deep, connected by a narrow neck. The most common benign tumour affecting the parotid gland is pleomorphic adenoma.

**Why the Correct Answer is Right:**
Pleomorphic adenoma is the most common benign tumour of the parotid gland due to the following reasons:

1. **Multifocal origin:** Pleomorphic adenoma arises from the ductal and stromal cells of the salivary gland, making it a bimolecular tumour.
2. **Epithelial and myoepithelial cell proliferation:** This mixture of cell types contributes to the diverse histological patterns seen in pleomorphic adenomas, which can make diagnosis challenging.
3. **Frequency:** Pleomorphic adenoma is the most common parotid gland tumour, accounting for around 80% of all benign salivary gland tumours.

**Why Each Wrong Option is Incorrect:**

A. **Warthin's tumour (parotid lymphoepithelial tumour):** This tumour primarily occurs in older adults and consists of a combination of epithelial and lymphoid tissue. While it can occur in the parotid gland, it is less common than pleomorphic adenoma.

B. **Salivary gland carcinoma:** Salivary gland carcinomas are malignant tumours that account for less than 10% of all parotid gland tumours. Pleomorphic adenoma is a distinct entity.

C. **Mucocele:** A mucocele is a benign cyst filled with mucus, usually caused by a blocked salivary gland duct. It is not a tumour but a cyst, making it unlikely as a correct answer.

D. **Mucocele:** Similar to option C, a mucocele is a benign cyst formed due to a blocked salivary gland duct. It is not a tumour but a cyst, making it an incorrect answer.

**Clinical Pearl:**
Understanding the difference between benign and malignant tumours is essential for accurate diagnosis and treatment planning. Pleomorphic adenoma is a benign tumour, while Warthin's tumour, salivary gland carcinoma, and mucocele are distinct entities. A thorough knowledge of these differences helps prevent misdiagnosis and inappropriate treatment strategies.