A wild zebra chase.

Rare diseases provide the physician with an exciting opportunity to escape from what may be perceived as boring routine. This endeavour, however, may hold dangers, particularly if the physician has never before encountered the disease in real life. All too easily, an open-mind approach is abandoned. From then on, new data will be used in a relentless quest to support a pre-formed diagnosis [1]. To illustrate this common and dangerous mechanism of error, we present the case of a 70-year-old patient with systemic lupus erythematosus with renal impairment and proteinuria, in whom renal biopsy seemed to provide us with a rare and interesting diagnosis.