A patient with recurrent pyogenic infections is found to have normal B and T cell counts but markedly reduced serum complement CH50. Specific analysis shows absent classical pathway activity but intact alternative pathway. Which complement component deficiency best explains this pattern?

• A C1q deficiency ✓
• B C3 deficiency
• C Properdin deficiency
• D C5 deficiency

Explanation

C1q deficiency selectively abrogates classical pathway activation while leaving the alternative and lectin pathways intact. CH50 measures the classical pathway (C1–C9) and would be absent, whereas AH50 (alternative pathway) would be normal. C1q deficiency also predisposes to SLE-like autoimmunity due to failure to clear apoptotic debris and immune complexes. C3 deficiency affects all three pathways. Properdin deficiency impairs the alternative pathway. C5 deficiency affects terminal lysis.

Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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