**Core Concept**
Erythema nodosum is a type of skin condition characterized by the formation of painful nodules or lesions, typically on the lower legs. It is often associated with underlying inflammatory or infectious processes, as well as certain medications or systemic diseases.

**Why the Correct Answer is Right**
The correct answer is associated with sarcoidosis, a systemic granulomatous disease that can affect multiple organs, including the lungs, skin, and lymph nodes. Erythema nodosum is a common skin manifestation of sarcoidosis, resulting from the formation of granulomas in the skin. The exact pathogenesis is unclear, but it is thought to involve an abnormal immune response, leading to inflammation and the characteristic skin lesions.

**Why Each Wrong Option is Incorrect**
**Option A:** Inflammatory bowel disease (IBD) can cause various skin manifestations, but erythema nodosum is not a common association. While IBD can cause skin lesions, these are typically different from erythema nodosum.
**Option B:** Rheumatoid arthritis is a systemic autoimmune disease that can cause skin manifestations, but erythema nodosum is not a typical association. Rheumatoid arthritis more commonly causes skin lesions such as nodules or vasculitis.
**Option C:** Leprosy is a chronic bacterial infection that can cause skin lesions, but erythema nodosum is not a characteristic skin manifestation. Leprosy typically causes skin lesions that are numb, discolored, or have a different texture.

**Clinical Pearl / High-Yield Fact**
Erythema nodosum is a type of panniculitis, which is inflammation of the subcutaneous fat. It is essential to consider sarcoidosis as a potential underlying cause of erythema nodosum, especially in patients with a history of lung disease or other systemic symptoms.

**Correct Answer:** C. Sarcoidosis.