A 75yo man presents with ARF (acute renal failure). He has been troubled by recurrent epistaxis but over the last 3wks he reports to have coughed up blood too. What is the single most likely positive antibody?

The key is B. C ANCA. [ARF and recurrent epistaxis and hemoptysis suggest Wegener’s granulomatosis. Wegener's granulometosis is a rare disease of uncertain cause that can affect people of all ages. It is characterized by inflammation in various tissues, including blood vessels (vasculitis), but primarily parts of the respiratory tract and the kidneys for which C ANCA is most specific].