A 22-year-old man has recurrent disseminated Neisseria meningitidis infections despite vaccination. Complement studies show undetectable CH50 but normal AH50. Which terminal complement component is most likely deficient?
- A C5 ✓
- B C3
- C C1q
- D C4
Correct answer: A. C5
Explanation
Terminal complement component deficiency (A5-A9) causes recurrent Neisseria infections because the membrane attack complex (MAC) cannot form. CH50 (classical pathway hemolytic activity) is low because A5 is downstream of the classical pathway. AH50 being normal here indicates the alternative pathway up to A5 is intact; the defect is in the terminal components. A1q or A4 deficiency would cause immune complex clearance defects, not Neisseria susceptibility.
Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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