Patient presenting with cutaneous vasculitis, glomerulonephritis, peripheral neuropathy, which investigation is to be performed next that will help you diagnose the condition?
**Core Concept**
Cutaneous vasculitis, glomerulonephritis, and peripheral neuropathy are manifestations of a systemic vasculitis, which is a group of disorders characterized by inflammation of the blood vessels. The underlying principle being tested is the association between these clinical features and the involvement of specific organs or systems.
**Why the Correct Answer is Right**
The investigation to be performed next is a test that can diagnose the underlying cause of the systemic vasculitis. The correct answer is a test that can detect autoantibodies against neutrophil cytoplasmic antigens (ANCA). ANCA-associated vasculitis is a type of systemic vasculitis that presents with glomerulonephritis, cutaneous vasculitis, and peripheral neuropathy. The presence of ANCA is a diagnostic criterion for this condition. The ANCA test can help identify the underlying cause of the systemic vasculitis and guide further management.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not directly diagnose the underlying cause of the systemic vasculitis. While a complete blood count (CBC) may show anemia or leukocytosis, it does not provide a specific diagnosis.
**Option B:** This option is incorrect because an ultrasound of the kidneys may show renal abnormalities, but it does not diagnose the underlying cause of the glomerulonephritis.
**Option C:** This option is incorrect because a skin biopsy may show vasculitis, but it does not provide a specific diagnosis of the underlying cause of the systemic vasculitis.
**Clinical Pearl / High-Yield Fact**
ANCA-associated vasculitis is a type of systemic vasculitis that presents with glomerulonephritis, cutaneous vasculitis, and peripheral neuropathy. The presence of ANCA is a diagnostic criterion for this condition, and the ANCA test is a crucial investigation to perform in patients with these clinical features.
**Correct Answer:** C. ANCA test