A 28-year-old woman of South Indian origin presents with nephrotic-range proteinuria (3.2 g/day), active urinary sediment with RBC casts, and rising serum creatinine. ANA is positive at 1:320, anti-dsDNA is elevated, and C3 + C4 are low. Renal biopsy shows diffuse proliferative glomerulonephritis with wire-loop thickening, endocapillary and extracapillary proliferation, and hyaline thrombi. The immunofluorescence pattern marked **D** in the diagram shows deposits of IgG, IgM, IgA, C3, and C1q. Which of the following best explains the diagnostic significance of this immunofluorescence pattern in lupus nephritis?