**Core Concept**
Russell bodies are characteristic inclusions found in the cells of individuals suffering from certain conditions, most notably in plasma cell dyscrasias. They are composed of immunoglobulin light chains, which accumulate within the endoplasmic reticulum due to a defect in the secretory pathway. This leads to the formation of large, eosinophilic inclusions that can be seen under the microscope.

**Why the Correct Answer is Right**
The intracellular accumulation of immunoglobulin light chains in plasma cells leads to the formation of Russell bodies. This occurs due to a defect in the protein secretory pathway, where the misfolded light chains accumulate within the endoplasmic reticulum. The endoplasmic reticulum's chaperone proteins, such as BiP (binding immunoglobulin protein), become overwhelmed and are unable to properly fold the light chains, resulting in their aggregation and formation of Russell bodies.

**Why Each Wrong Option is Incorrect**
**Option A:** While misfolded proteins can accumulate in various cellular compartments, Russell bodies are specifically formed from immunoglobulin light chains within the endoplasmic reticulum.
**Option B:** The presence of Russell bodies is not a characteristic feature of viral infections, although some viral infections can disrupt protein synthesis and lead to the accumulation of misfolded proteins.
**Option C:** The accumulation of amyloid fibrils is a distinct process that occurs in certain conditions, such as Alzheimer's disease and systemic amyloidosis, and is not directly related to Russell body formation.

**Clinical Pearl / High-Yield Fact**
Russell bodies are a hallmark feature of plasma cell dyscrasias, such as multiple myeloma, and can be seen under the microscope in affected cells. They are composed of immunoglobulin light chains and are a useful diagnostic clue in the differential diagnosis of plasma cell disorders.

**Correct Answer: C. Immunoglobulin light chains.