A 32-year-old woman from rural Maharashtra presents with progressive dyspnea, bilateral hilar lymphadenopathy, and erythema nodosum. Chest X-ray shows non-caseating granulomas on biopsy. All of the following are true regarding the pathogenesis of this condition EXCEPT:

Explanation

## Sarcoidosis: Non-Caseating Granulomatous Inflammation **Key Point:** The clinical presentation (dyspnea, hilar lymphadenopathy, erythema nodosum, non-caseating granulomas) is diagnostic of **sarcoidosis**, a systemic granulomatous disease that characteristically does NOT show caseating necrosis. ### Sarcoidosis vs. Tuberculosis: Granuloma Morphology | Feature | Sarcoidosis | Tuberculosis | |---------|------------|---------------| | Granuloma type | Non-caseating | Caseating | | Central necrosis | Absent | Cheese-like caseous necrosis | | Epithelioid cells | Present | Present | | Langhans giant cells | Present | Present | | Acid-fast bacilli | Absent | Present (Ziehl-Neelsen) | | Clinical presentation | Systemic (lungs, skin, eyes) | Pulmonary, extrapulmonary | | Prognosis | Often self-limited | Progressive if untreated | **High-Yield:** Caseating necrosis is the **hallmark of tuberculosis**, NOT sarcoidosis. This is a critical differentiating feature on histology. The absence of caseation in sarcoidosis is one of the diagnostic criteria. ### Pathogenesis of Sarcoidosis (Correct Features) 1. **Th1/Th17-mediated response:** IFN-γ and TNF-α activate macrophages 2. **Epithelioid transformation:** Macrophages become epithelioid cells 3. **Giant cell formation:** Fusion of epithelioid cells → Langhans and foreign body giant cells 4. **Fibrotic encapsulation:** Chronic phase with fibroblast infiltration and scarring **Clinical Pearl:** Erythema nodosum (painful nodules on anterior shins) is a common systemic manifestation of sarcoidosis, representing a hypersensitivity reaction to sarcoid antigens. It indicates better prognosis than systemic sarcoidosis without skin involvement. **Mnemonic:** **SAR**coidosis = **SAR**coplasmic (non-caseating), **TB** = **TB**erculous (caseating). ```mermaid flowchart TD A[Granulomatous disease on biopsy]:::outcome --> B{Caseating necrosis present?}:::decision B -->|Yes| C[Tuberculosis]:::outcome B -->|No| D[Consider sarcoidosis, fungal, or other non-caseating causes]:::outcome D --> E{AFB stain positive?}:::decision E -->|Yes| F[Mycobacterial infection]:::outcome E -->|No| G[Sarcoidosis, berylliosis, or hypersensitivity pneumonitis]:::outcome ``` [cite:Robbins 10e Ch 3]