D

Flow-cytometric immunophenotyping of peripheral blood is the standard first-line test for severe combined immunodeficiency (SCID): characteristic “T-, B-, NK±” patterns of lymphocyte subsets allow rapid recognition of the various genetic forms before molecular confirmation [Kanegane, 2018, PMID 28684198; Knight, 2019, PMID 31069989]. Bruton disease, Hyper-IgM syndrome and Bare-lymphocyte syndrome can also use flow cytometry, but their diagnosis relies primarily on specific protein or immunoglobulin measurements, whereas SCID is defined—and routinely screened—in practice by its flow-cytometry lymphocyte pattern.