**Question:** The feature of Collapsing glomerulopathy is:

A. Proliferation of mesangial cells and expansion of mesangial matrix
B. Disruption of the glomerular basement membrane (GBM)
C. Increased intraglomerular pressure leading to podocyte foot process effacement
D. Collapsing of the glomerular capillary loops due to podocyte foot process fusion and effacement

**Core Concept:** Collapsing glomerulopathy is a term used to describe a group of glomerular diseases characterized by the collapse of the capillary loops in the glomerulus, primarily affecting the podocytes (the cells lining the capillary loops). This leads to the fusion and effacement of podocyte foot processes, resulting in impaired filtration and proteinuria.

**Why the Correct Answer is Right:** In Collapsing glomerulopathy, the primary event is the fusion and effacement of podocyte foot processes. This leads to the collapse of the glomerular capillary loops as the foot processes are the structural units responsible for maintaining the integrity of the glomerular filtration barrier. The resulting proteinuria and hematuria are the hallmark clinical features of this condition.

**Why Each Wrong Option is Incorrect:**

A. Mentioning mesangial cell proliferation and expansion of mesangial matrix is incorrect because it does not directly relate to the primary event, which is podocyte foot process fusion and effacement.

B. Disruption of the glomerular basement membrane (GBM) is not the primary event in collapsing glomerulopathy. Although some cases may have concurrent GBM damage, the focus should be on podocyte alterations.

C. Increased intraglomerular pressure is a consequence or secondary effect of podocyte dysfunction, not the primary event. Increased pressure may exacerbate the condition, but it is not the cause of the collapsing process.

**Clinical Pearl:** Collapsing glomerulopathy is most often associated with certain viral infections, such as HIV and hepatitis B, or autoimmune conditions like lupus nephritis. It is crucial for students to understand the primary event in this condition, as this knowledge is essential for clinical reasoning and differential diagnosis when encountering patients with proteinuria and hematuria.