A 6-year-old boy has recurrent Neisseria infections and a CH50 that is essentially zero; classical and alternative pathway components are individually normal. The most likely deficiency is:

• A C3 deficiency
• B Properdin deficiency
• C Mannose-binding lectin deficiency
• D Terminal complement component deficiency (C5–C9) ✓

Explanation

Terminal complement component deficiencies (C5 through C9 — the membrane attack complex components) result in a near-zero CH50 because the haemolytic assay requires full complement lysis, yet AH50 (alternative pathway) and individual components are normal. These patients are selectively susceptible to Neisseria meningitidis and N. gonorrhoeae bacteremia because intact MAC is critical for killing encapsulated Neisseria. C3 deficiency would profoundly impair opsonization and predispose to multiple encapsulated organisms. Properdin deficiency only impairs the alternative pathway amplification loop. MBL deficiency causes mild susceptibility to infections in infancy.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.