**Core Concept**
Membranoproliferative glomerulonephritis (MPGN) is a type of glomerulonephritis characterized by the proliferation of mesangial cells and the thickening of the glomerular basement membrane (GBM) due to the deposition of immune complexes. This results in a characteristic splitting of the GBM, which is a hallmark of the disease.

**Why the Correct Answer is Right**
The splitting of the GBM in MPGN is due to the deposition of immune complexes, such as IgG and C3, on both sides of the GBM, leading to its thickening and proliferation of mesangial cells. This process is mediated by the activation of the complement system and the subsequent activation of mesangial cells, which produce extracellular matrix proteins and contribute to the thickening of the GBM. The splitting of the GBM is a result of the insertion of mesangial cells and matrix between the original GBM.

**Why Each Wrong Option is Incorrect**
**Option A:** Subepithelial deposits are characteristic of membranous nephropathy, not MPGN.
**Option B:** Foamy cells are typically seen in conditions such as nephrotic syndrome, where there is an accumulation of lipids in the podocytes.
**Option D:** Subendothelial deposits are characteristic of post-streptococcal glomerulonephritis, not MPGN.

**Clinical Pearl / High-Yield Fact**
MPGN is often associated with conditions such as systemic lupus erythematosus, hepatitis C, and cryoglobulinemia, highlighting the importance of a thorough evaluation of the patient's medical history and laboratory results in the diagnosis of MPGN.

**✓ Correct Answer: C. Splitting of glomerular basement membrane**