**Core Concept**
Streptococcal Glomerulonephritis (SGN) is an immune complex-mediated glomerulonephritis that occurs as a complication of streptococcal infections, particularly pharyngitis or skin infections. The pathogenesis involves the deposition of antigen-antibody complexes in the glomeruli, leading to an inflammatory response.

**Why the Correct Answer is Right**
The correct answer is **C. Blood smear showing hematuria and red blood cell casts**. This is because the presence of hematuria (blood in the urine) and red blood cell casts in the urine is a hallmark of glomerulonephritis. The presence of these findings on a blood smear is indicative of glomerular damage and inflammation. The kidney's glomeruli filter the blood and form urine, and damage to these structures can cause red blood cells and proteins to leak into the urine.

**Why Each Wrong Option is Incorrect**
* **Option A:** Elevated serum creatinine levels are a nonspecific indicator of renal dysfunction and can be seen in various conditions, including acute tubular necrosis, pyelonephritis, and other causes of kidney damage. While elevated serum creatinine levels may be seen in SGN, they are not specific for this condition.
* **Option B:** A positive throat culture for Group A beta-hemolytic streptococcus may indicate a recent streptococcal infection, but it is not a direct indicator of glomerulonephritis. Many individuals with streptococcal infections do not develop SGN.
* **Option D:** A low complement level may be seen in SGN, but it is not a specific or sensitive marker for this condition. Complement levels can be low in various other conditions, including systemic lupus erythematosus and other autoimmune diseases.

**Clinical Pearl / High-Yield Fact**
The presence of hematuria and red blood cell casts in the urine is a classic finding in glomerulonephritis, including SGN. This is because the glomeruli are damaged, allowing red blood cells and proteins to leak into the urine.

**Correct Answer:** C. Blood smear showing hematuria and red blood cell casts