Complement deficiencies — branching viva

Outpatient clinic: a previously well fourteen-year-old boy presents for review after his second episode of invasive meningococcal septicaemia in two years. He is entirely well between episodes, with normal growth and no other infections. His maternal uncle died of meningococcal meningitis in childhood. C3 and C4 are normal but the CH50 is undetectable and the AP50 is normal. 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 and family screening — then branches to a twelve-year-old with angioedema without urticaria that did not respond to adrenaline and asks you to justify the correct therapy.