**Core Concept**
The nephron's filtration barrier is a crucial structure that allows for the selective filtration of blood components, enabling the kidney to excrete waste products while retaining essential nutrients and ions. This barrier consists of three layers: the endothelial cells of the glomerular capillaries, the fenestrated podocytes, and the glomerular basement membrane.

**Why the Correct Answer is Right**
The glomerular basement membrane (GBM) is a thick, non-fenestrated layer composed of collagen, laminin, and other glycoproteins that provides mechanical support to the filtration barrier. The GBM acts as a selective barrier, allowing small molecules like water, ions, and glucose to pass through while restricting larger molecules like proteins and blood cells.

**Why Each Wrong Option is Incorrect**
**Option A:** The endothelial cells of the glomerular capillaries are fenestrated, allowing for the free passage of small molecules and fluids. They are an essential component of the filtration barrier.
**Option B:** The podocytes are highly specialized cells with foot processes that wrap around the glomerular capillaries, creating a filtration slits that allow small molecules to pass through while restricting larger molecules.
**Option C:** The mesangium is a supporting tissue that provides structural support to the glomerular capillaries and helps to regulate the filtration process.

**Clinical Pearl / High-Yield Fact**
The glomerular filtration rate (GFR) is a critical indicator of kidney function, and its measurement is essential for diagnosing and monitoring kidney diseases.

**Correct Answer: C. The mesangium does not form a filtration barrier in nephrons. It provides structural support to the glomerular capillaries.**