A 17-month-oldgin presented with history of recurrent attacks of pneumonia since the age of 8 months. You suspect a predominant B-cell defect. Of the following, the BEST simple initial screening test is

(A). If the lgA level is normal, selective lgA deficiency, which is the most common B~ cell defect, is excluded, as are most of the permanent types of hypogammaglobulinemia, as lgA is usually very low or absent in those conditions. if lgA is low, lgG and lgM should also be measured.