**Question:** Mixed tumours of the salivary glands are:

**Core Concept:** Salivary gland tumours are a diverse group of neoplasms that can be classified based on their histopathological (tissue) features. Mixed tumours are a type of salivary gland tumour that consists of two or more histological types of cells.

**Why the Correct Answer is Right:**
Mixed tumours of the salivary glands are a rare type of salivary gland neoplasm. They are also known as pleomorphic adenomas or mixed epithelial and stromal tumours, reflecting their composition of epithelial (cellular) and stromal (connective tissue) components. This type of tumour arises from the interaction between the epithelial and stromal cell components, which together form the functional units of the salivary gland, called acini.

**Why Each Wrong Option is Incorrect:**
A. Salivary gland oncocytomas: These tumours are composed of cells with abundant, eosinophilic cytoplasm, which is different from mixed tumours.
B. Pleomorphic adenomas: This is a synonym for mixed tumours, but it is not a separate entity; the correct term is mixed epithelial and stromal tumours.
C. Mucoepidermoid carcinomas: These tumours are malignant (cancerous) and composed of cells of epithelial and mucus-producing types, which is different from mixed tumours.
D. Warthin's tumours: These are benign lymphocytic tumours caused by the coexistence of a lymphoid nodule and a mucus-producing acinus, which is distinct from mixed tumours.

**Correct Answer:** D. Warthin's tumours (also known as lymphoepithelial tumours) are benign lymphocytic tumours that occur primarily in the parotid gland and are characterized by the coexistence of a lymphoid nodule and a mucus-producing acinus. In contrast to mixed tumours, Warthin's tumours are not composed of both epithelial and stromal components.

**Explanation:**

Mixed tumours of the salivary glands are characterized by the coexistence of epithelial and stromal components. The epithelial component may include basaloid, squamous, or ductal cells, while the stromal component is typically composed of myoepithelial cells and extracellular matrix. These tumours are benign, slow-growing lesions with a well-defined border and a capsule.

**Why Each Wrong Option is Incorrect:**
A. Salivary gland oncocytomas: These tumours are benign (not malignant) and are composed of cells with abundant mitochondria, which is different from mixed tumours.
B. Pleomorphic adenomas: These are benign tumours composed of both epithelial and myoepithelial components, not mixed tumours.
C. Mucoepithelial tumours: These are benign tumours composed of both mucus-producing and epithelial cells, not mixed tumours, which are composed of both epithelial and stromal components.
D. Warthin's tumours