**Core Concept**
Benign salivary gland tumors, particularly pleomorphic adenomas, are the most common cause of unilateral parotid swelling in adults, often presenting as a painless, slow-growing mass.

**Why the Correct Answer is Right**
Pleomorphic adenomas arise from the parotid gland's ductal or acinar epithelium and are characterized by their ability to differentiate into both epithelial and mesenchymal components. They typically present in the superficial lobe of the parotid gland and are often associated with a firm, painless mass that moves with the facial nerve. The exact cause of pleomorphic adenomas is unknown, but they are thought to result from a combination of genetic and environmental factors.

**Why Each Wrong Option is Incorrect**
* **Option A:** Mucoceles, while possible in the parotid gland, are less common and typically present as a fluctuant, painless mass filled with mucin. They are more commonly associated with the submandibular gland.
* **Option B:** Sjögren's syndrome is an autoimmune disease characterized by lymphocytic infiltration of the salivary glands, leading to dry mouth and dry eyes. While it can cause bilateral parotid swelling, it is not the most common cause of unilateral parotid swelling.
* **Option C:** Parotiditis, or parotid gland inflammation, can be caused by viral infections such as mumps or bacterial infections. However, it is typically associated with pain and tenderness, rather than a painless mass.

**Clinical Pearl / High-Yield Fact**
When evaluating a patient with a unilateral parotid mass, it is essential to differentiate between benign and malignant tumors. A thorough history, physical examination, and imaging studies (such as ultrasound or CT scans) can help guide further diagnosis and treatment.

**Correct Answer: C. Parotiditis.