**Core Concept**
Lupus nephritis is a common complication of systemic lupus erythematosus (SLE), characterized by inflammation and damage to the kidneys' glomeruli. Wire loop lesions are a hallmark pathologic finding in lupus nephritis, resulting from immune complex deposition and subsequent capillary wall thickening.

**Why the Correct Answer is Right**
Wire loop lesions are formed due to the deposition of immune complexes, specifically antinuclear antibodies (ANAs) and complement, within the glomerular capillary walls. This leads to the activation of the complement system, resulting in the influx of neutrophils and macrophages, which in turn cause the thickening of the capillary walls. The resulting appearance under the microscope resembles a wire loop, hence the name. The thickening of the capillary walls impairs the glomerular filtration barrier, leading to proteinuria and hematuria.

**Why Each Wrong Option is Incorrect**

**Option A:** Basement membrane thickening is a characteristic feature of membranous nephropathy, not lupus nephritis. While basement membrane changes can occur in lupus nephritis, they are not the primary cause of wire loop lesions.

**Option B:** Basement membrane thickening is the hallmark of membranous nephropathy, not lupus nephritis. In lupus nephritis, the immune complexes deposit within the capillary walls, not the basement membrane.

**Option C:** Subepithelial deposits are a feature of membranoproliferative glomerulonephritis (MPGN), not lupus nephritis. In lupus nephritis, the deposits are typically found in the capillary walls, not the subepithelial space.

**Option D:** Sclerosis of mesangium is a feature of various glomerular diseases, including IgA nephropathy and focal segmental glomerulosclerosis (FSGS), but not lupus nephritis. Wire loop lesions are specific to lupus nephritis and are characterized by capillary wall thickening, not mesangial sclerosis.

**Clinical Pearl / High-Yield Fact**
Wire loop lesions are a classic histopathological feature of lupus nephritis and are often associated with a poor prognosis. Early recognition of these lesions is crucial for the diagnosis and management of lupus nephritis.

**✓ Correct Answer: A. Capillary wall thickening**