**Question:** Which of the following diseases show fibrinoid necrosis?

A. Rheumatoid arthritis
B. Systemic lupus erythematosus (SLE)
C. Polyarteritis nodosa
D. Sjögren's syndrome

**Core Concept:**
Fibrinoid necrosis is a pathological process characterized by the degradation and fragmentation of fibrin into fibrin degradation products (FDPs) within vessel walls. This results in the formation of fibrinoid material that can cause vasculitis and damage to blood vessel walls.

**Why the Correct Answer is Right:**
The correct answer, Polyarteritis nodosa (PAN), is a systemic necrotizing vasculitis that primarily affects small and medium-sized arteries, arterioles, and capillaries. Fibrinoid necrosis is a hallmark of PAN, as it leads to the damage of blood vessel walls and subsequent clinical features.

**Why Each Wrong Option is Incorrect:**
A. Rheumatoid arthritis is an autoimmune disorder characterized by synovial inflammation and joint destruction. While it can cause vasculitis, fibrinoid necrosis is not a defining feature of this condition.
B. Systemic lupus erythematosus (SLE) is an autoimmune disease characterized by widespread inflammation. Vasculitis can occur in SLE, but fibrinoid necrosis is not a defining feature of the disease.
D. Sjögren's syndrome is an autoimmune disorder primarily affecting the exocrine glands. Although Sjögren's can cause vasculitis, fibrinoid necrosis is not a key feature of this condition.

**Clinical Pearl:**
Understanding fibrinoid necrosis is crucial for recognizing and diagnosing diseases involving vasculitis, such as Polyarteritis nodosa. Fibrinoid necrosis can help differentiate between various vasculitic diseases.