Complement deficiencies — formative SAQs

SAQ 1 (10)

A previously well fourteen-year-old boy is admitted with his second episode of invasive meningococcal septicaemia in two years. He recovered fully from the first episode and has been entirely well between admissions, with no other significant infections and normal growth. His maternal uncle died of meningococcal meningitis in childhood. His C3 and C4 are normal, but the total haemolytic complement (CH50) is undetectable, and the AP50 is normal. ^{[9] [10]}

a) What is the most likely category of diagnosis, and which pathway is affected? Explain why the child is "otherwise well" despite the recurrent invasive infection. (3 marks) ^[9]

b) Justify why a normal C3 and C4 does not exclude this diagnosis, and name the correct first-line screen. (2 marks) ^{[1] [2]}

c) Outline the further investigations needed to confirm the specific deficiency and to plan family screening. (2 marks) ^[5]

d) Discuss the principles of long-term management, including prevention, family implications, and prognosis. (3 marks) ^{[9] [10]}