**Core Concept**
Glomerular proteinuria results from damage to the glomerular filtration barrier, particularly the glomerular basement membrane or podocytes, leading to loss of protein in urine. It is typically caused by diseases that directly affect glomerular structure or function, such as diabetic nephropathy or systemic amyloidosis.

**Why the Correct Answer is Right**
Multiple myeloma primarily causes **tubular proteinuria** due to the deposition of monoclonal immunoglobulins (M-proteins) in the renal tubules, not glomerular damage. While it can lead to renal complications, these are mostly tubular in origin. In contrast, diabetes mellitus (A) causes glomerular basement membrane thickening and podocyte injury, amyloidosis (B) involves amyloid protein deposition in glomeruli, and "nil lesion" (D) implies no pathology — thus, no proteinuria. However, D is misleading as it suggests no cause, so it's not a valid pathological cause. The key distinction is that multiple myeloma does not primarily cause glomerular proteinuria.

**Why Each Wrong Option is Incorrect**
Option A: Diabetes Mellitus causes glomerular damage via hyperglycemia-induced podocyte injury and glomerulosclerosis, leading to glomerular proteinuria.
Option B: Amyloidosis deposits amyloid fibrils in glomeruli, disrupting filtration and causing glomerular proteinuria.
Option D: "Nil lesion" implies no pathology, so it cannot be a cause of proteinuria — thus, it is not a valid cause, making it a distractor. However, the question asks for "not a cause," and D is not a disease, so it is logically invalid.

**Clinical Pearl / High-Yield Fact**
Remember: **Glomerular proteinuria** is due to glomerular injury; **tubular proteinuria** is due to tubular damage. In multiple myeloma, proteinuria is typically **tubular**, not glomerular. Always assess the proteinuria type to differentiate underlying pathology.

✓ Correct Answer: C. Multiple myeloma