A genetically inherited X-linked error in metabolism caused by an inadequate amount of the enzyme glucose-6-phosphate dehydrogenase in red blood cells. When exposed to certain environmental influences, such as fava beans and some drugs (sulphonamide antibiotics, some antimalarials including primaquine, and several other sulphur-containing drugs) , the red cells of individuals with this deficiency burst, resulting in severe anemia. As in the case of sickle-cell trait and thalassemia, this anemia may provide some immunity to falciparum malaria. G6PD deficiency is also known as "favism. " It is the most common type of human enzyme deficiency--about 400 million people have this condition around the world. It is a common problem especially in populations around the Mediterranean Basin, the Middle East, and Southeast Asia.