**Core Concept**
IgA nephropathy, also known as Berger's disease, is a kidney disease characterized by the deposition of IgA antibodies in the glomeruli. This condition leads to inflammation and damage to the glomerular capillaries, ultimately impairing kidney function.

**Why the Correct Answer is Right**
The hallmark of IgA nephropathy is the presence of mesangial IgA deposits, which are detected by immunofluorescence microscopy. These deposits are composed of IgA1 molecules that are aberrantly glycosylated and prone to aggregation. The deposition of these IgA1 molecules in the mesangium triggers an inflammatory response, leading to the recruitment of inflammatory cells and the release of various cytokines and chemokines.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because IgA nephropathy is not characterized by the deposition of IgG antibodies. While IgG antibodies may be present in some cases, the hallmark of IgA nephropathy is the presence of IgA antibodies.

**Option B:** This option is incorrect because IgA nephropathy is not typically associated with systemic lupus erythematosus (SLE). While both conditions can involve the kidneys, they have distinct pathophysiological mechanisms and clinical presentations.

**Option C:** This option is incorrect because IgA nephropathy is not typically associated with diabetic nephropathy. While both conditions can cause kidney damage, they have distinct pathophysiological mechanisms and clinical presentations.

**Clinical Pearl / High-Yield Fact**
The presence of IgA deposits in the mesangium is a key diagnostic feature of IgA nephropathy. This can be confirmed by immunofluorescence microscopy, which shows a characteristic "punctate" or "starry sky" pattern of IgA deposition.

**Correct Answer:** C. Mesangial IgA deposits