Paroxysmal nocturnal hemoglobinuria (PNH) is a rare, acquired blood disorder characterized by complement-induced breakdown of red blood cells in the bloodstream. This leads to anemia, red urine, and an increased risk of blood clots. PNH results from a genetic defect causing a lack of protective proteins on red blood cells and other cell surfaces, making them targets for destruction by the immune system's complement system. Treatment options include long-term anticoagulation therapy to prevent clots, transfusions to address anemia, and the monoclonal antibody eculizumab which inhibits complement system activity and reduces symptoms.