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A 3-year-old boy is being evaluated for persistent diarrhea. Although the patient seemed healthy at his 12-month well child visit, since then he has experienced 4 episodes of otitis media and 3 episodes of pneumococcal pneumonia. He was at the 50th percentile for weight and height at 12 months but is now at the 25th percentile for height and 10th percentile for weight. The patient is referred for upper gastrointestinal endoscopy, and Giardia lamblia is isolated from duodenal aspirates. Further workup shows very low serum levels of all immunoglobulin types . Flow cytometry of this patient's peripheral blood is most likely to show a near absence of cells bearing which of the following markers?