**Core Concept**
Bilateral parotid gland enlargement can be caused by various conditions, including infectious, inflammatory, and neoplastic processes. The parotid glands are the largest salivary glands, located on either side of the face, and their enlargement can be a manifestation of systemic or local diseases.

**Why the Correct Answer is Right**
Bilateral parotid gland enlargement is commonly seen in **Sjögren's syndrome**, an autoimmune disorder characterized by lymphocytic infiltration and destruction of the exocrine glands, including the salivary and lacrimal glands. This leads to symptoms of dry mouth (xerostomia) and dry eyes (xerophthalmia). The enlargement of the parotid glands in Sjögren's syndrome is due to the chronic inflammation and lymphoid hyperplasia within the gland. **Option C** is correct because Sjögren's syndrome is a well-established cause of bilateral parotid gland enlargement.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not provided, so we cannot evaluate its correctness.

**Option B:** While **mumps** can cause parotid gland enlargement, it typically presents unilaterally and is less likely to cause bilateral enlargement. Mumps is a viral infection caused by the mumps virus, which leads to inflammation of the parotid glands.

**Option D:** **Diabetes mellitus** can cause xerostomia due to decreased salivary gland function, but it typically does not cause bilateral parotid gland enlargement.

**Clinical Pearl / High-Yield Fact**
Sjögren's syndrome can be associated with other autoimmune disorders, such as rheumatoid arthritis, lupus, and thyroiditis, and patients with Sjögren's syndrome are at increased risk of developing lymphoma.

**Correct Answer:** C. Sjögren's syndrome.