A patient with a rare complement deficiency presents with recurrent Neisseria meningitidis infections. CH50 is low but alternative pathway activity is intact. Which terminal complement component is most likely deficient?

• A C5 ✓
• B C3
• C C1q
• D C9

Explanation

Deficiencies of terminal complement components (A5, A6, A7, A8, A9) predispose specifically to recurrent Neisseria infections because the membrane attack complex (MAC) is essential for killing these organisms. A5 deficiency is the most common terminal component deficiency associated with this susceptibility. A1q and A3 deficiencies cause broader pyogenic infections and immune complex disease. A9 deficiency is often asymptomatic or mild because A5b-8 can form a partial lytic complex.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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