A 7-year-old boy presents with bilateral swelling around his eyes. His parents state that the child’s eyes have become “puffy” over the past several weeks, and his urine has become cocoa-colored. Physical examination reveals bilateral periorbital edema, but peripheral edema is not found. The boy is afebrile and his blood pressure is slightly elevated. A urinary dipstick reveals mild proteinuria, while microscopic examination of the boy’s urine reveals hematuria with red blood cell casts. Laboratory tests reveal increased ASO titers and decreased serum C3 levels, but C2 and C4 levels are normal. A throat swab for streptococci is negative. A microscopic section from the kidney reveals increased numbers of cells within the glomeruli. An electron microscopic section of the kidney reveals large electron-dense deposits in the glomeruli that are located between the basement membrane and the podocytes. The foot processes of the podocytes are otherwise unremarkable. Which one of the following renal diseases most likely produced the abnormalities in this young boy?

**Core Concept**
Post-streptococcal glomerulonephritis (PSGN) is an immune complex-mediated disease characterized by the deposition of antigens and antibodies in the glomeruli, leading to inflammation and damage. It typically follows an infection with nephritogenic strains of Group A beta-hemolytic streptococci.

**Why the Correct Answer is Right**
The boy's presentation with periorbital edema, proteinuria, hematuria with red blood cell casts, and decreased serum C3 levels is consistent with PSGN. The increased ASO titers indicate a recent streptococcal infection, which is the common precipitating factor for PSGN. The electron microscopic finding of large electron-dense deposits in the glomeruli, located between the basement membrane and the podocytes, is characteristic of immune complex deposition in PSGN. The normal levels of C2 and C4 complement components suggest that the complement pathway is not activated through the classical pathway, which is consistent with the role of immune complexes in PSGN.

**Why Each Wrong Option is Incorrect**
**Option A:** Minimal Change Disease (MCD) typically presents with nephrotic syndrome (heavy proteinuria, hypoalbuminemia, and edema) and normal kidney biopsy findings. The presence of hematuria with red blood cell casts and decreased serum C3 levels in this case is not consistent with MCD.
**Option B:** Focal Segmental Glomerulosclerosis (FSGS) often presents with nephrotic syndrome and a characteristic "collapsing" lesion on biopsy. The electron microscopic findings in this case do not support a diagnosis of FSGS.
**Option C:** Membranoproliferative Glomerulonephritis (MPGN) typically involves the deposition of immune complexes or complement components in the glomeruli, but the electron microscopic findings in this case do not support a diagnosis of MPGN.
**Option D:** IgA Nephropathy (Berger's disease) often presents with hematuria, but the presence of red blood cell casts and decreased serum C3 levels in this case is not consistent with IgA Nephropathy.

**Clinical Pearl / High-Yield Fact**
PSGN often follows a streptococcal infection, such as strep throat or skin infections, and may present with symptoms such as periorbital edema, hematuria, and proteinuria. The diagnosis is supported by the presence of elevated ASO titers and decreased serum C3 levels.

**Correct Answer:** C. Post-streptococcal glomerulonephritis.

✓ Correct Answer: A. Acute post-streptococcal glomerulonephritis