**Question:** A 24-year-old woman presents with abdominal pain, joint discomfort, and lower limb rash. She was well until 1 week before presentation. On examination, she has a palpable purpuric rash on her legs, nonspecific abdominal discomfort, and no active joints. She has 3+proteinuria, normal WBC, no eosinophils, and elevated creatinine of 1.6 mg/dL. Biopsy of the rash confirms vasculitis with immunoglobulin A (IgA) 3+ and C3 (complement 3) deposition on immunofluorescence. For the above patient with vasculitis syndrome, select the most likely diagnosis.

A. Henoch-Schönlein Purpura
B. Polyarteritis Nodosa
C. IgA Nephropathy
D. Rheumatoid Arthritis

**Correct Answer:** A. Henoch-Schönlein Purpura

**Core Concept:**
Henoch-Schönlein Purpura (HSP) is a type of vasculitis, a condition characterized by inflammation of blood vessels. It is a childhood disease but can rarely affect adults like in the presented case. In HSP, small-sized blood vessels, particularly capillaries and venules, become inflamed, leading to the clinical manifestations seen in the patient.

**Why the Correct Answer is Right:**
In this case, the clinical findings, biopsy results, and laboratory parameters help us identify the patient as having HSP.

1. **Clinical Features:** The patient presents with abdominal pain (epigastric, right hypochondriac, and left iliac fossa pain), joint discomfort, and purpuric rash on lower limbs. These features are characteristic of HSP.
2. **Biopsy Results:** The biopsy shows a palpable purpuric rash, which is a hallmark feature of HSP. Additionally, the biopsy demonstrates vasculitis with IgA deposition, which further confirms the diagnosis. IgA is a type of immunoglobulin present in the blood and tissues. Its deposition in the blood vessels indicates an immune response, leading to vasculitis.
3. **Laboratory Parameters:** The patient has elevated creatinine (1.6 mg/dL), which is a sign of glomerular involvement. Proteinuria (3+) and microscopic hematuria (3+) indicate renal involvement, another common feature in HSP.

**Why Other Options are Incorrect:**
1. **Diffuse Arthritis (Option D):** The patient has joint discomfort but no active arthritis, making this option incorrect.
2. **Polyarteritis Nodosa (Option B):** Although the patient has renal involvement, the abdominal pain is not prominent, and there is no characteristic palpable purpuric rash, making this option incorrect.
3. **IgA Nephropathy (Option C):** The patient has elevated creatinine, proteinuria, and hematuria, indicating renal involvement. However, HSP is a systemic disease affecting various organs, including the kidneys, joints, and skin. The absence of arthritis, abdominal pain