Antibody deficiencies — branching viva

Outpatient clinic: a seven-month-old boy presents with his second lobar pneumonia this year, both requiring intravenous antibiotics. His maternal uncle died in infancy. On examination his tonsils are absent and there are no palpable lymph nodes. Total IgG, IgA and IgM are all markedly low. The examiner asks: what is your framework, what is the most likely diagnosis, how will you confirm it, and what are the principles of long-term management — then branches to a thriving toddler with a single low IgG and asks you to justify why you would NOT start immunoglobulin.