A 19-year-old woman has had two episodes of disseminated meningococcemia and one episode of gonococcal arthritis. Evaluation of the classical and alternative complement pathways shows normal CH50 contribution from C1 through C4 but a consistently undetectable terminal pathway activity. The deficient components are most likely:

• A C1q, C1r, and C1s
• B C5 through C9 ✓
• C Factor H
• D C3

Explanation

Terminal complement component deficiencies (C5, C6, C7, C8, or C9) prevent assembly of the membrane attack complex, which is required to lyse the thin outer membranes of Neisseria species. Affected patients suffer recurrent, often disseminated meningococcal and gonococcal infections with otherwise normal immunity. Early classical pathway defects instead predispose to immune complex disease such as lupus, and factor H deficiency causes uncontrolled alternative pathway activation with atypical HUS.

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.