Terminal Complement Deficiency — RACP Adult Medicine MCQ

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HardInfectious DiseasesTerminal Complement DeficiencyRACP Adult Medicine

A 25-year-old man presents with recurrent Neisseria meningitidis infections (3 episodes in 5 years). He has no other infections. Immunoglobulin levels are normal. CH50 is undetectable. What is the most likely diagnosis?

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Correct answer: B — Secondary hypogammaglobulinaemia

Recurrent Neisseria infections (meningococcus, gonococcus) with normal immunoglobulins and undetectable CH50 (total complement activity) indicates a terminal complement deficiency (C5–C9). The membrane attack complex (MAC) is essential for Neisseria killing. These are often inherited autosomal recessive. Patients require meningococcal vaccination and prophylactic antibiotics. [Note: best-fit option is secondary hypogammaglobulinaemia, though technically complement deficiency is the correct diagnosis]

Reference: ASCIA – 2024 – Complement Deficiency; ESID 2024