PLE-3579

PLE-3579

A 2-year-old boy is brought to the physician because of fever and listlessness for 12 hours. He has had recurrent episodes of pneumonia and otitis media over the past year. Two maternal uncles died of pneumonia in early childhood. One year ago, he was at the 50th percentile for height and weight; he is currently at the 25th percentile for height and 10th percentile for weight. He appears ill. His temperature is 39 C (102.2 F), blood pressure is 60/40 mm Hg, pulse is 160/min, and respirations are 36/min. Examination shows cool and mottled extremities. A blood culture grows Streptococcus pneumoniae. Serum IgE, IgG, and IgM levels are markedly decreased. Which of the following diagnostic tests is most likely to be abnormal?