The protein in glomerular basement membrane responsible for charge dependent filtration is
**Core Concept**
The glomerular basement membrane (GBM) is a selective filter that allows the passage of small molecules and ions while restricting the movement of larger molecules. Charge-dependent filtration is a critical mechanism that separates negatively charged molecules from positively charged ones.
**Why the Correct Answer is Right**
The GBM contains negatively charged molecules, primarily glycoproteins and proteoglycans, that contribute to its negative charge. This charge-dependent filtration is mediated by the presence of negatively charged sites on the GBM that repel negatively charged molecules, such as albumin and other proteins, thereby preventing their passage into the urine. The negatively charged sites on the GBM also attract positively charged molecules, facilitating their filtration.
**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because it does not specify the protein responsible for charge-dependent filtration. While the GBM does contain glycoproteins, it is not a specific protein that is directly responsible for this mechanism.
**Option B:** Incorrect because it is a component of the GBM, but not the primary protein responsible for charge-dependent filtration. The GBM is composed of type IV collagen, laminin, and other glycoproteins and proteoglycans.
**Option C:** Incorrect because it is not a protein found in the GBM. While podocalyxin is a negatively charged protein, it is primarily found on the surface of podocytes and is not responsible for charge-dependent filtration in the GBM.
**Clinical Pearl / High-Yield Fact**
The GBM's negative charge is critical for maintaining the selective permeability of the glomerular filtration barrier, and alterations in this charge can lead to proteinuria and kidney disease.
**Correct Answer:** C. Podocalyxin is not correct because it was not listed among the options.