A 19-year-old woman has had two episodes of meningococcemia. Screening for immunodeficiency shows normal immunoglobulin levels, normal lymphocyte subsets, and normal neutrophil function. Which investigation is most likely to reveal the underlying defect?

• A Measurement of total serum hemolytic complement (CH50) ✓
• B Nitroblue tetrazolium dye reduction test
• C Flow cytometry for CD40 ligand expression
• D Quantitative serum IgG subclass levels

Explanation

Deficiency of terminal complement components C5 through C9 predisposes specifically to recurrent infections by Neisseria species because membrane attack complex formation is required for bacteriolysis of these organisms. Immunoglobulins and cellular immunity are intact, so CH50 screening for classical pathway terminal activity is the correct test. Nitroblue tetrazolium screens chronic granulomatous disease, which causes catalase-positive organism abscesses rather than recurrent Neisseria sepsis.

Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.