A 7-year-old boy is brought to the clinic by his parents after developing red urine earlier in the day. The patient has asthma, allergic rhinitis, and atopic dermatitis. He was recently treated for a skin infection. Blood pressure is 140/90 mm Hg. On physical examination, there is periorbital edema as well as pitting edema on both feet. Laboratory results are as follows: Which of the following is most likely responsible for this patient's renal injury?

Question

A 7-year-old boy is brought to the clinic by his parents after developing red urine earlier in the day.  The patient has asthma, allergic rhinitis, and atopic dermatitis.  He was recently treated for a skin infection.  Blood pressure is 140/90 mm Hg.  On physical examination, there is periorbital edema as well as pitting edema on both feet.  Laboratory results are as follows:  Which of the following is most likely responsible for this patient's renal injury?

Quizplus · Accepted Answer

11ec160b_37e1_f8a8_9249_bf2beb9f56c3_MD0001_00 This patient with atopic dermatitis is predisposed to secondary skin infections (eg, impetigo, cellulitis). His antecedent skin infection, along with nephritic syndrome, suggests poststreptococcal glomerulonephritis (PSGN), the most common form of acute nephritis in children. During infection, antibodies form against antigens expressed by nephritogenic strains of group A beta-hemolytic Streptococcus (eg, S pyogenes). These antistreptococcal antibodies combine with streptococcal antigens to form immune complexes that are deposited along the glomerular basement membrane (type III hypersensitivity). These deposits can then be visualized as electron-dense subepithelial "humps" on electron microscopy and as granular depositions within the mesangium and glomerular capillary walls on IgG and C3 immunofluorescence. (Choices A and E) Cytotoxic CD8⁺ T lymphocytes and macrophages play a prominent role in type IV (delayed-type) hypersensitivity reactions. These cells are stimulated by T helper cells, leading to localized inflammation, cellular destruction, and granuloma formation. Type IV hypersensitivity is responsible for contact dermatitis (eg, poison ivy) and positive tuberculin skin test reactions. (Choice B) Histamine is released by mast cells and basophils during type I (immediate) hypersensitivity, which is seen in anaphylaxis and allergies, not in PSGN. Mast cells and basophils are coated by IgE molecules, which cross-link on antigen exposure, triggering the release of histamine and other mediators. (Choice C) Autoantibodies are responsible for type II (cytotoxic) hypersensitivity reactions. This reaction is responsible for Goodpasture syndrome (antibodies directed at type IV collagen), which can cause nephritic syndrome and hemoptysis. However, it is rare in children and is not associated with recent streptococcal infection. Other type II hypersensitivity reactions include autoimmune hemolytic anemia, immune thrombocytopenic purpura, and pemphigus vulgaris. Educational objective: Poststreptococcal glomerulonephritis is the most common cause of nephritic syndrome (eg, hematuria, edema, hypertension) in children, typically occuring 2-4 weeks after a streptococcal infection (eg, impetigo, cellulitis, pharyngitis). It is caused by a type III (immune-complex-mediated) hypersensitivity reaction resulting from nephritogenic strains of group A beta-hemolytic Streptococcus.

A 7-Year-Old Boy Is Brought to the Clinic by His