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C2 Deficiency — RACP Adult Medicine MCQ

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HardInfectious DiseasesC2 DeficiencyRACP Adult Medicine

A 2-year-old child presents with recurrent encapsulated bacterial infections (S. pneumoniae, H. influenzae) and an SLE-like syndrome (photosensitive rash, arthritis, positive ANA). C2 is undetectable. CH50 is low. C3, C4, and C1 esterase inhibitor are normal. What is the most likely immunodeficiency?

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Correct answer: D — C2 deficiency

Recurrent encapsulated bacterial infections PLUS SLE-like autoimmune disease with undetectable C2, low CH50, and normal C3/C4/C1-INH is C2 deficiency — the most common inherited complement deficiency. Early classical pathway components (C1q, C2, C4) are associated with SLE-like autoimmunity (impaired immune complex clearance) and susceptibility to encapsulated organisms. Management: vaccination, prompt antibiotic treatment for infections.

Reference: ASCIA – 2024 – Complement Deficiency; ESID 2024