C-ANCA positivity indicates, antibody formed against-
**Core Concept**
C-ANCA positivity is associated with a specific autoimmune disorder characterized by the presence of autoantibodies against a particular enzyme. This condition is known for causing vasculitis and affecting various organs in the body.
**Why the Correct Answer is Right**
C-ANCA positivity indicates the presence of antibodies formed against proteinase 3 (PR3), a neutrophil enzyme. These autoantibodies are directed against the PR3 enzyme, which is located in the azurophilic granules of neutrophils. The formation of these autoantibodies leads to the activation of neutrophils, resulting in the release of granules and the subsequent inflammation and damage to blood vessels and organs.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because C-ANCA positivity is not associated with antibodies formed against myeloperoxidase (MPO), which is a different enzyme and is actually associated with P-ANCA positivity.
**Option B:** This option is incorrect because C-ANCA positivity is not associated with antibodies formed against elastase, although elastase is also a neutrophil enzyme.
**Option C:** This option is incorrect because C-ANCA positivity is not associated with antibodies formed against lactoferrin, which is a protein found in neutrophil granules but is not the target of C-ANCA autoantibodies.
**Clinical Pearl / High-Yield Fact**
In patients with C-ANCA positivity, the most commonly affected organs are the kidneys, lungs, and nerves. The presence of C-ANCA autoantibodies is a strong indicator of granulomatosis with polyangiitis (GPA), a type of vasculitis characterized by the presence of granulomas and inflammation in the affected organs.
**Correct Answer:** C. Proteinase 3 (PR3)