**Core Concept**
Post-streptococcal glomerulonephritis (PSGN) is an immune complex-mediated glomerulonephritis that occurs after a streptococcal infection, typically pharyngitis or skin infection. This condition is characterized by the deposition of immune complexes containing antigens from the streptococcal bacteria and antibodies produced by the host, leading to inflammation and damage of the glomeruli.

**Why the Correct Answer is Right**
The diagnosis of PSGN is primarily based on the detection of streptococcal antigens or antibodies in the serum. The most commonly used test is the anti-streptolysin O (ASO) titer, which measures the levels of antibodies against streptolysin O, a toxin produced by group A beta-hemolytic streptococci. Elevated ASO titers suggest a recent streptococcal infection, which can be the trigger for PSGN. Other tests, such as the anti-streptokinase titer and the streptococcal DNAase B titer, may also be used to support the diagnosis.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as it does not specify a diagnostic test for PSGN. While serum creatinine may be elevated in PSGN, it is not a diagnostic test for the condition.
**Option B:** This option is incorrect as it refers to a condition that is not directly related to PSGN. IgA nephropathy is a different type of glomerulonephritis that presents with hematuria and proteinuria, but it is not a diagnostic criterion for PSGN.
**Option C:** This option is incorrect as it refers to a treatment for PSGN, not a diagnostic test. Corticosteroids may be used to reduce inflammation and prevent progression of the disease, but they are not used for diagnosis.

**Clinical Pearl / High-Yield Fact**
A key clinical clue to PSGN is the presence of a recent streptococcal infection, which can be identified by elevated ASO titers. The diagnosis of PSGN should be considered in children who present with hematuria, proteinuria, and elevated serum creatinine after a streptococcal infection.

**Correct Answer: C. Corticosteroids may be used to reduce inflammation and prevent progression of the disease, but they are not used for diagnosis.