A young woman with recurrent Neisseria meningitidis infections despite vaccination is evaluated for complement deficiency. Her CH50 is zero. Which component is most likely deficient if the AP50 (alternative pathway haemolysis) is also zero?

• A C3 ✓
• B C1q
• C Properdin
• D Factor D

Explanation

C3 deficiency abolishes both the classical and alternative pathways because C3 is the convergence point for all complement activation routes; hence both CH50 and AP50 are zero. C1q deficiency abolishes only the classical pathway (CH50 zero, AP50 normal). Properdin and Factor D deficiencies impair only the alternative pathway (AP50 low, CH50 may be normal). Terminal complement (C5–C9) deficiency predisposes specifically to Neisseria, but CH50 is zero while AP50 remains normal.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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