A 4 year-old boy, develops a large erythematous rash around the site of a mosquito bite. One month later, he is taken to a pediatrician because of a puffy face and swollen ankles. The scanty urine sample has a reddish-brown hue, and contains both red blood cells and protein. Which of the following distinctive features would be most likely to be seen on renal biopsy?

**Core Concept**
The patient's symptoms are suggestive of post-streptococcal glomerulonephritis (PSGN), a kidney disease that occurs after a streptococcal infection, such as strep throat or skin infections. PSGN is characterized by inflammation of the glomeruli, the filtering units of the kidneys.

**Why the Correct Answer is Right**
The correct answer is related to the pathophysiology of PSGN. In PSGN, the immune system mistakenly attacks the glomeruli, leading to the deposition of immune complexes in the glomerular basement membrane. This results in the activation of the complement system, which causes damage to the glomeruli and leads to the symptoms of kidney disease. The distinctive feature that would be most likely to be seen on renal biopsy is the presence of subepithelial humps, which are deposits of immune complexes in the glomerular basement membrane.

**Why Each Wrong Option is Incorrect**
**Option A:** Acute tubular necrosis (ATN) is a type of kidney injury that occurs due to ischemia or nephrotoxins, and is not typically associated with PSGN. ATN would show damage to the renal tubules on biopsy.
**Option B:** membranous nephropathy is a type of kidney disease that is characterized by the deposition of immune complexes in the glomerular basement membrane, but it is not typically associated with PSGN. Membranous nephropathy would show a different pattern of immune complex deposition on biopsy.
**Option C:** Focal segmental glomerulosclerosis (FSGS) is a type of kidney disease that is characterized by scarring of the glomeruli, but it is not typically associated with PSGN. FSGS would show a different pattern of scarring on biopsy.

**Clinical Pearl / High-Yield Fact**
In PSGN, the symptoms of kidney disease typically develop 1-3 weeks after the initial streptococcal infection, and are often accompanied by a low platelet count and elevated levels of serum complement.

**Correct Answer:** C. Subepithelial humps

✓ Correct Answer: D. Subepithelial electron dense humps