Paroxysmal Nocturnal Hemoglobinuria (PNH) is a rare hemolytic anemia caused by a mutation in the PIGA gene, which disrupts the production of protective proteins (CD55 and CD59) on red blood cells. Without these proteins, red blood cells become vulnerable to complement-mediated destruction, leading to hemolysis, thrombosis, and anemia. Hemoglobin released from destroyed cells leads to dark urine, particularly in the morning. PNH can be diagnosed using flow cytometry to detect missing proteins and treated with complement inhibitors like eculizumab to prevent further destruction.