**Core Concept**
The question is testing knowledge of the most common type of glomerulonephritis, which is a group of kidney diseases characterized by inflammation of the glomeruli, the filtering units of the kidneys. This condition can lead to impaired kidney function and is often associated with various underlying causes, including autoimmune diseases, infections, and vascular disorders.

**Why the Correct Answer is Right**
The most common form of glomerulonephritis is IgA nephropathy (Berger's disease), which is characterized by the deposition of IgA antibodies in the glomeruli. This condition often presents with hematuria (blood in the urine) and can progress to chronic kidney disease. IgA nephropathy is thought to result from an abnormal immune response, leading to the production of IgA antibodies that are deposited in the glomeruli, causing inflammation and damage.

**Why Each Wrong Option is Incorrect**
**Option A:** Focal Segmental Glomerulosclerosis (FSGS) is a common cause of nephrotic syndrome, but it is not the most common form of glomerulonephritis.
**Option B:** Membranous nephropathy is another common cause of nephrotic syndrome, but it is not the most common form of glomerulonephritis.
**Option C:** Rapidly Progressive Glomerulonephritis (RPGN) is a severe form of glomerulonephritis, but it is not the most common form.

**Clinical Pearl / High-Yield Fact**
IgA nephropathy is often associated with symptoms such as hematuria, proteinuria, and hypertension, and can be triggered by various factors, including infections, trauma, and stress. Early diagnosis and treatment can help slow the progression of the disease.

**Correct Answer:** IgA nephropathy is the most common form of glomerulonephritis.