Primary immunodeficiency with rheumatic manifestations — branching viva

Outpatient clinic: a six-year-old girl presents with a three-month symmetric polyarthritis labelled juvenile idiopathic arthritis, but she has also had two pneumonias this year, chronic diarrhoea, and bruising from a platelet count of 40 × 10⁹/L, and her total IgG is low. The examiner asks: what is your framework, what is the most likely diagnosis, how will you confirm it, what are the principles of management and the hazard of a biologic — then branches to an eight-year-old boy with a granulomatous colitis indistinguishable from Crohn disease plus a staphylococcal liver abscess and a Serratia pneumonia, and asks you to name the test that settles it, and finally to a five-year-old boy with severe childhood lupus and a persistently absent CH50 and asks you to explain the mechanism.