A 14-year-old girl presents with a 5-day history of hypertension, oliguria, and hematuria. She was seen 2 weeks earlier for a severe throat infection with group A (b-hemolytic) streptococci. A kidney biopsy displays glomerulonephritis. Immunofluorescence staining for which of the following proteins would provide the strongest evidence that this patient's glomerulonephritis is mediated by immune complexes?

In acute postinfectious glomerulonephritis, immune complexes localize in glomeruli by deposition from the circulation or by formation in situ as bacterial antigens bind circulating antibodies. The renal biopsy shows complement fixation. Complement activation is so extensive that over 90% of patients with postinfectious glomerulonephritis develop hypocomplementemia. Complement and other inflammatory mediators attract and activate neutrophils and monocytes, which stimulate the proliferation of mesangial and endothelial cells, resulting in diffuse proliferative glomerulonephritis. Typically, the level of serum C3 is depressed during the acute syndrome but returns to normal within 1 to 2 weeks. The other choices involve the coagulation system and are not components of immune complexes.Diagnosis: Postinfectious glomerulonephritis, nephritic syndrome