Question

A case of recurrent Neisseria gonorrhoeae infection is being evaluated. Which immunological investigation would best detect a complement pathway defect?

• A. C1 esterase inhibitor assay
• B. Quantitative immunoglobulin levels
• C. Nitroblue tetrazolium test
• D. Terminal complement (C5--C9) assay

Hint:

Recurrent Neisseria infections always point toward terminal complement deficiency, especially C5--C9 deficiency.

Answer 1

The Correct Option is D

Concept: Recurrent infections with Neisseria species are classically associated with deficiency of the terminal complement pathway, especially C5 to C9. These components form the membrane attack complex.

Step 1: Identify the organism.
The patient has recurrent Neisseria gonorrhoeae infection. Recurrent infections by Neisseria suggest a defect in complement-mediated bacterial killing.

Step 2: Recall the role of terminal complement.
The terminal complement components C5, C6, C7, C8, and C9 form: \[ \text{Membrane Attack Complex (MAC)} \] MAC is important for killing Gram-negative bacteria, especially Neisseria.

Step 3: Select the best investigation.
To detect a defect in this pathway, the best test is: \[ \text{Terminal complement } (C5-C9) \text{ assay} \] Therefore, the correct answer is Terminal complement (C5--C9) assay.