**Core Concept**
Membranous glomerulonephritis (MGN) is a type of kidney disease characterized by the thickening of the glomerular basement membrane (GBM), which is composed of a thin, uniform layer of extracellular matrix. This thickening is often due to the deposition of immune complexes and other substances on the epithelial side of the GBM.

**Why the Correct Answer is Right**
In MGN, the primary mechanism is the deposition of immune complexes on the epithelial side of the GBM. These immune complexes are composed of antibodies and antigens that are not normally present in the kidney. Over time, the accumulation of these immune complexes leads to the activation of complement and the production of cytokines, which in turn cause the thickening of the GBM. This thickening can lead to the loss of glomerular function and the onset of nephrotic syndrome. The characteristic subepithelial deposits seen on electron microscopy in MGN are a result of the accumulation of these immune complexes on the epithelial side of the GBM.

**Why Each Wrong Option is Incorrect**
**Option B:** Focal glomerular sclerosis is a type of kidney disease characterized by the scarring of individual glomeruli, but it does not involve the thickening of the GBM or the deposition of immune complexes.

**Option C:** Rapidly progressive glomerulonephritis (RPGN) is a type of kidney disease characterized by a rapid decline in renal function, but it is not typically associated with the deposition of immune complexes on the epithelial side of the GBM.

**Option D:** Minimal change disease (MCD) is a type of kidney disease characterized by the loss of foot processes in the podocytes, but it does not involve the thickening of the GBM or the deposition of immune complexes.

**Clinical Pearl / High-Yield Fact**
MGN is often associated with the presence of autoantibodies, such as antinuclear antibodies (ANA) or anti-smooth muscle antibodies (ASMA), which can be detected in the serum of affected individuals. This association highlights the importance of considering autoimmune mechanisms in the pathogenesis of MGN.

**✓ Correct Answer: A. Membranous glomerulonephritis (MGN)**