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Terminal Complement Deficiency — RACP Adult Medicine MCQ

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ModerateInfectious DiseasesTerminal Complement DeficiencyRACP Adult Medicine

A 20-year-old man has recurrent episodes of Neisseria meningitidis meningitis (3 episodes in 4 years). Immunoglobulins are normal. CH50 (total haemolytic complement) is undetectable. AH50 is also undetectable. C3 is normal but C5 is undetectable. What is the most likely immunodeficiency?

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Correct answer: D — Terminal complement deficiency (C5-C9)

Recurrent Neisseria infections with undetectable CH50, undetectable AH50, normal C3, and absent C5 is terminal complement pathway deficiency (C5-C9). The membrane attack complex (MAC) is essential for killing Neisseria species. Inherited as autosomal recessive. Management: meningococcal vaccination (ACWY + B), prophylactic antibiotics, medical alert identification, and education about early antibiotic use for febrile illness.

Reference: ASCIA – 2024 – Complement Deficiency; ESID 2024