**Core Concept**
Erythema nodosum (EN) is a type of skin inflammation characterized by the formation of painful nodules or lesions, typically on the lower extremities. It is often associated with idiopathic causes, but can also be linked to various underlying medical conditions, including infections, medications, and autoimmune disorders.

**Why the Correct Answer is Right**
Erythema nodosum is a form of panniculitis, which involves inflammation of the subcutaneous fat tissue. The exact pathogenesis of EN is not fully understood, but it is thought to involve an immune-mediated response, with the activation of neutrophils and the release of pro-inflammatory cytokines. This leads to the formation of nodules, which can be painful and tender to the touch.

**Why Each Wrong Option is Incorrect**
**Option A:**
Erythema nodosum can indeed be associated with tuberculosis (TB), particularly in regions where TB is endemic. In fact, TB is one of the classic causes of EN, and the two conditions often co-exist.

**Option B:**
Sarcoidosis is another condition that can cause EN. Sarcoidosis is a systemic granulomatous disease that can affect multiple organs, including the lungs, skin, and lymph nodes. The association between sarcoidosis and EN is well-documented, and both conditions often present with similar symptoms.

**Option C:**
Bacterial infections, such as streptococcal pharyngitis or skin infections, can also trigger EN. The exact mechanism is not fully understood, but it is thought to involve an immune-mediated response to the bacterial infection.

**Clinical Pearl / High-Yield Fact**
When evaluating a patient with EN, it is essential to consider the possibility of an underlying systemic condition, such as TB or sarcoidosis. A thorough medical history, physical examination, and diagnostic workup are critical in determining the underlying cause of EN.

**Correct Answer: D.**