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SLE — SCE Rheumatology MCQ

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HardAutoimmune SerologySLESCE Rheumatology

A 30-year-old woman with SLE has remained clinically quiescent for 12 months. Her renal function, urine protein:creatinine ratio and anti-dsDNA titre are stable. On three appropriately collected and promptly processed samples, C4 is undetectable but C3 is normal. Classical-pathway functional activity (CH50) is absent, whereas alternative-pathway activity (AP50) is normal. Her sister, who also has SLE, has the same complement profile. Which underlying explanation is most likely?

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Correct answer: C — Inherited early classical-pathway complement deficiency

The correct answer is C. An absent CH50 with preserved AP50 localises the defect to the early classical pathway; undetectable C4 with normal C3 specifically supports an inherited C4 or related early classical-component deficiency. Such deficiencies impair immune-complex and apoptotic-cell clearance and strongly predispose to SLE. The identical stable profile in an affected sibling and the absence of clinical or serological disease activity further favour inheritance over consumption. Chronic infection or occult active SLE would more typically cause variable complement consumption rather than a reproducible, pathway-selective familial pattern. Acquired anti-complement antibodies can cause hypocomplementaemia but would not usually produce the same stable profile in a sibling. Correct collection and prompt processing make pre-analytical complement degradation unlikely. Individual component measurement and genetic analysis would confirm the defect.

Reference: Brodszki N et al. European Society for Immunodeficiencies and ERN RITA Complement Guideline: Deficiencies, Diagnosis, and Management, 2020. https://pubmed.ncbi.nlm.nih.gov/36535812/