a case report was published describing the evaluation of a 6-year-old girl born with an antibody-deficiency syndrome. her parents were consanguineous (closely related) and of moroccan descent. flow-cytometric analysis of her blood cells revealed that the girl had b cells, because cell-surface igm and cd20 were detected. her b cells, however, did not express surface cd19. a homozygous defect in which of the following genes would be a plausible underlying cause of this deficiency?