Paroxysmal nocturnal hemoglobinuria. A rare disorder in which red blood cells are easily destroyed by certain immune system proteins. Symptoms include blood clots, and red or brownish urine in the morning. Aplastic anemia (decreased production of blood cells) may lead to PNH, and people with this disorder are at increased risk of acute myelogenous leukemia. Also called paroxysmal nocturnal hemoglobinuria.