## **Core Concept**
Churg-Strauss Syndrome (CSS) and Wegener's Granulomatosis (WG) are both forms of vasculitis, specifically antineutrophil cytoplasmic antibody (ANCA)-associated vasculitides. They share some clinical features but also have distinct differences, particularly in their associations with asthma and eosinophilia.

## **Why the Correct Answer is Right**
The correct answer, **Eosinophilia**, is more frequently seen in Churg-Strauss Syndrome compared to Wegener's Granulomatosis. Churg-Strauss Syndrome is characterized by asthma, eosinophilia, and vasculitis. Eosinophilia is a hallmark feature that distinguishes CSS from other forms of vasculitis, including WG. The presence of eosinophilia and asthma are key diagnostic criteria for CSS.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While both conditions involve inflammation and can affect the lungs, the specific feature of **Eosinophilia** is more distinctive of CSS.
- **Option B:** **Renal involvement** is common in both conditions but is not the distinguishing feature here; it's more about the type of involvement rather than the presence.
- **Option C:** **Cavitation** can occur in both diseases but is less typical for CSS; it's actually more characteristic of WG.
- **Option D:** This option might seem plausible because both diseases can have **ANCA positivity**, but the type (MPO-ANCA more common in CSS and PR3-ANCA in WG) and clinical context differ.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that Churg-Strauss Syndrome is often associated with **asthma** and **eosinophilia**, making **eosinophilia** a critical distinguishing feature from Wegener's Granulomatosis. Remember, CSS is one of the small vessel vasculitides characterized by the triad of asthma, eosinophilia, and vasculitis.

## **Correct Answer:** B. Eosinophilia.