An 8-year-old boy presents with periorbital edema and throbbing headaches. His parents report that the boy had a "strep throat" 2 weeks ago. Urinalysis shows 3+ hematuria. A renal biopsy shows hypercellular glomeruli, and electron microscopic examination of glomeruli discloses subepithelial "humps." Which of the following best explains the pathogenesis of glomerulonephritis in this patient?

Deposition of circulating immune complexes. Type III hypersensitivity reactions are characterized by immune complex deposition, complement fixation, and localized inflammation. Antibody directed against either a circulating antigen or an antigen that is deposited in a tissue can give rise to a type III response. Diseases that seem to be most clearly attributable to the deposition of immune complexes are systemic lupus erythematosus, rheumatoid arthritis, and varieties of glomerulonephritis. Streptococcal infection in this case led to the deposition of antigens and antibodies in glomerular basement membranes, resulting in clinical features of nephritic syndrome (e.g., hematuria, oliguria, and hypertension). Post-streptococcal illnesses do not include any of the other choices.Diagnosis: Postinfectious glomerulonephritis