Terminal Complement Deficiency — RACP Adult Medicine MCQ

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HardInfectious DiseasesTerminal Complement DeficiencyRACP Adult Medicine

A 60-year-old man presents with recurrent Neisseria meningitidis septicaemia (his second episode). His complement studies show absent total haemolytic complement (CH50) activity. What complement deficiency is most likely?

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Correct answer: C — C5 deficiency

Recurrent meningococcal infections are strongly associated with deficiency of terminal complement components (C5–C9) that form the membrane attack complex (MAC). C5 deficiency is the most commonly tested. Absent CH50 confirms a complement pathway defect. All patients with recurrent Neisseria infections should be tested. They should receive meningococcal vaccination (ACWY + B) and may require antibiotic prophylaxis. Eculizumab (anti-C5) therapy also creates a similar susceptibility.

Reference: eTG – 2025 – Immunology; ASCIA – 2024 – Complement Deficiency Guidelines