**Core Concept**
Membranous glomerulonephritis (GN) is a type of kidney disease characterized by the deposition of immune complexes on the glomerular basement membrane. This leads to a thickening of the basement membrane and an increase in proteinuria. The complement system plays a crucial role in the pathogenesis of membranous GN, particularly in the activation of the classical pathway.

**Why the Correct Answer is Right**
In membranous GN with reduced complement level, the complement system is often depleted due to chronic activation and consumption. This is often seen in cases of chronic infection or autoimmune disease. The reduction in complement levels can lead to further immune complex deposition and exacerbation of the disease. The classical pathway of the complement system is activated by the binding of C1q to the Fc region of IgG antibodies, which are often present in membranous GN.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because membranous GN with reduced complement level is not typically associated with IgA nephropathy, which is characterized by the deposition of IgA antibodies in the glomeruli.
**Option B:** This option is incorrect because membranous GN with reduced complement level is not typically associated with minimal change disease, which is characterized by the loss of foot processes of podocytes and normal complement levels.
**Option C:** This option is incorrect because membranous GN with reduced complement level is not typically associated with diabetic nephropathy, which is characterized by the deposition of mesangial matrix and nodular glomerulosclerosis.

**Clinical Pearl / High-Yield Fact**
Reduced complement levels in membranous GN can be a marker of chronic disease and increased risk of progression to end-stage renal disease.

**Correct Answer:** C. Membranous GN with reduced complement level is seen in systemic lupus erythematosus.