Granulomatosis with Polyangiitis, also known as Wegener's Granulomatosis or just Wegener's is a rare and deadly form of vasculitis. It is distinguished from other forms of vasulitis by the presence of granulomas and certain blood tests. The most determinant blood test is the C-ANCA.
Wegener's is extremely difficult to diagnose as its symptoms in the initial phase are not distinguishable from the symptoms of many other common diseases. These include symptoms such as aches, difficulty breathing, sinus congestion, and chest pain. It is generally only diagnosed after many other common diseases have been ruled out. As such, because of its rarity and indistinguishable symptoms, it is considered one of the ultimate zebra diagnoses.
Wegener's is a frequent differential on the show, but it only appeared at the end of Season 7, where is was almost mistaken for cancer. This was not a stretch as both conditions respond well to chemo therapy and the granulomas can be mistaken for tumors, particularly where they cannot be biopsied.
Until the widespread development of steroids and immunosuppressants, Wegener's was a virtual death sentence with most patients dying within a year of diagnosis. However, modern steroids and immunosupressents can control the disease, although they cannot cure it. Most patients now live a normal life span, although they may suffer from flare-ups of symptoms from time to time.