A 28-year-old woman with lupus nephritis is found on serological testing to have low C3, low C4, normal CH50 activity, and markedly elevated anti-dsDNA titres. Which component of the complement pathway is most likely being consumed preferentially in this patient?
- A The alternative pathway factors B, D and properdin
- B The lectin pathway MBL-MASP complexes exclusively
- C The classical pathway components C1q, C4, C2 and C3 ✓
- D The terminal complement components C5–C9 (MAC)
Correct answer: C. The classical pathway components C1q, C4, C2 and C3
Explanation
Immune complex deposition in SLE preferentially activates the classical pathway, consuming C1q, C4, C2 and C3; thus both C3 and C4 fall together. The CH50 measures the functional lytic capacity of the entire complement cascade and will also decrease when upstream components are depleted. The alternative pathway (low C3 with normal C4) is consumed in conditions such as membranoproliferative GN type II or infective endocarditis. Normal CH50 with low C3 but normal C4 would suggest alternative pathway activation or isolated C3 deficiency.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.