**Question:** Linear pattern of Ig deposition in glomerular basement membrane is seen in

A. IgA nephropathy
B. Membranoproliferative glomerulonephritis (MPGN)
C. Membranous nephropathy
D. IgM nephropathy

**Correct Answer:** C. Membranous nephropathy

**Core Concept:**
Glomerulonephritis is a group of diseases that affect the glomeruli, small blood vessels in the kidneys responsible for filtering blood and maintaining fluid balance. Linear deposition of immunoglobulins (proteins) in the glomerular basement membrane (GBM) is a key feature of several types of glomerulonephritis.

**Why the Correct Answer is Right:**
The correct answer, Membranous nephropathy (C), is characterized by the deposition of immunoglobulins (mainly IgG) and complement proteins (C3) in the subepithelial space of the GBM, leading to a distinct "salt-and-pepper" appearance on kidney biopsy. This results in increased permeability of the GBM, leading to proteinuria, hematuria, and sometimes hypertension.

**Why Each Wrong Option is Incorrect:**
A. IgA nephropathy (IgA) is characterized by deposition of IgA in mesangium (center of the glomerulus), causing mesangioproliferative changes and often associated with gastrointestinal symptoms (e.g., hemosiderosis).
B. Membranoproliferative glomerulonephritis (MPGN) is characterized by both subepithelial and mesangial deposition of immunoglobulins and complement proteins, resulting in increased permeability and inflammation. It can be secondary to infections, autoimmune conditions, or drug reactions.
D. IgM nephropathy is characterized by deposition of IgM in mesangium, leading to mesangioproliferative changes and nephrotic syndrome.

**Clinical Pearl:**
Membranous nephropathy is crucial to understand because it is a common cause of nephrotic syndrome and proteinuria, which can lead to complications like hypertension, renal dysfunction, and, rarely, end-stage renal disease. Early diagnosis and treatment can prevent progression to end-stage renal disease.