A 12-year-old boy has a cough and earache for the past 2 days. He has a history of recurrent infections, including otitis media, diarrhea, and pneumonia. Physical examination shows an erythematous right tympanic membrane, a cleft palate, and murmur suggestive of congenital heart disease. A thoracic CT scan shows a small thymus. Results of laboratory studies suggest mild hypoparathyroidism. Which of the following diagnostic studies is most likely to be helpful in diagnosing this patient's condition?

DiGeorge syndrome is an immunodeficiency characterized by infection, a small thymus, congenital malformations, and hypoparathyroidism. This cluster is characteristic of the 22q11.2 deletion syndrome, readily diagnosed by FISH. Adenosine deaminase deficiency can cause immunodeficiency, but it is not associated with congenital malformations. Branched DNA assay can detect HIV infection that can lead to AIDS, but no congenital anomalies are associated with this condition. A lymph node biopsy may show a reduction in T cells or B cells associated with various forms of immunodeficiency, but this is not a specific test that can aid in confirming a specific diagnosis. Trinucleotide repeats of the X chromosome, detected by PCR, are seen in fragile X syndrome, which manifests with mental retardation in males.