T-cell and combined immunodeficiencies — branching viva

Opening question

Examiner: Take me through this infant. What does the low TREC result mean, and what is your frame for managing him? ^[1]

Candidate: A low T-cell receptor excision circle result on the newborn screen means this infant has few naive T cells leaving the thymus, which raises SCID as the leading diagnosis. My frame is that SCID is a medical emergency — it is fatal within the first one to two years without curative therapy — so I need to confirm the diagnosis with flow cytometry, protect the infant from iatrogenic harm while the diagnosis is secured, and refer urgently to a transplant centre, because the timing of haematopoietic stem cell transplantation — before infection and ideally before 3.5 months of age — is the single most powerful determinant of survival. ^[1]

Examiner: The family history is interesting. Why does it matter? ^[1]

Candidate: A maternal uncle who died in infancy before a diagnosis was made is the classic pedigree for X-linked SCID. The IL2RG mutation is carried by the mother, and affected male infants on the maternal line present and die without a diagnosis if newborn screening is not available. This history, combined with the low TREC result and the low lymphocyte count, makes X-linked SCID the leading diagnosis, and flow cytometry is likely to show the T⁻B⁺NK⁻ pattern. ^[1]