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?
Adenosine deaminase assay in lymphocytes Branched DNA assay for HIV-1 RNA level FISH analysis with a probe for chromosome 22q11.2 Lymph node biopsy