Atypical Hemolytic Uremic Syndrome Caused by a Rare Complement Factor B Mutation.

Thrombotic microangiopathy (TMA), a rare and diagnostically challenging condition, commonly presents with a triad of thrombocytopenia, hemolytic anemia, and end-organ damage, such as renal failure. Most cases of the hemolytic uremic syndrome (HUS) are mediated by Shiga toxin-producing Escherichia coli, but some cases present as an atypical HUS, which includes thrombotic thrombocytopenic purpura and complement-mediated thrombotic microangiopathy (C-TMA). Although C-TMA occurs because of genetic and acquired mutations in the complement regulatory factors, it is usually hereditary. The currently available treatment options include therapeutic plasma exchange and administration of eculizumab, which is a monoclonal antibody against C5. Here, we report a diagnostically challenging and extremely rare case of a middle-aged Caucasian man who was diagnosed with atypical HUS that was caused by a mutation in complement factor B. This case highlights the importance of not overlooking rare causes of TMAs because the diagnostic evaluation is important for guiding appropriate management and obtaining a favorable prognosis.