## **Core Concept**
The question assesses knowledge of dermatological manifestations of systemic diseases, specifically vasculitides. Palpable purpura is a key physical examination finding that points towards a particular diagnosis. This condition involves inflammation of blood vessels, which can lead to skin lesions.

## **Why the Correct Answer is Right**
The correct answer, **Henoch-Schönlein Purpura (HSP)**, is a form of vasculitis that affects blood vessels and is characterized by the deposition of IgA immune complexes. It commonly presents with palpable purpura, arthritis, abdominal pain, and renal involvement. The palpable purpura typically start on the legs and buttocks. HSP is the most common form of vasculitis in children, making it a likely diagnosis in a 7-year-old boy presenting with these symptoms.

## **Why Each Wrong Option is Incorrect**
- **Option A: Atopic dermatitis** is incorrect because, while it can present with skin lesions, it typically involves eczematous changes rather than palpable purpura. Atopic dermatitis often affects the face, neck, and flexural areas.
- **Option B: Erythema multiforme** is incorrect as it presents with target-shaped lesions, not typically palpable purpura. It can be triggered by infections or medications.
- **Option D: Idiopathic thrombocytopenic purpura (ITP)** is incorrect because, although it causes purpura, the purpura are not palpable. ITP is characterized by isolated thrombocytopenia leading to mucocutaneous bleeding.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that Henoch-Schönlein Purpura (HSP) often follows a gastrointestinal infection, particularly with certain strains of bacteria. A classic presentation includes the tetrad of palpable purpura, abdominal pain, arthritis, and renal involvement. Early recognition can prevent complications, especially renal involvement.

## **Correct Answer: C. Henoch-Schönlein Purpura.**