This video discusses intravascular hemolytic anemias, focusing on paroxysmal nocturnal hemoglobinuria (PNH). PNH occurs due to an acquired defect in myeloid stem cells, leading to a lack of protective proteins like DAF (CD55) and MIRL (CD59) on red blood cells. Without these, activated complement attacks blood cells, causing hemolysis primarily at night due to acidosis. This results in hemoglobin in the blood and urine. PNH also increases the risk of thrombosis and acute myeloid leukemia. The diagnosis involves tests like sucrose screening and flow cytometry.