A 74-year-old man presents to the clinic for assessment of symptoms of fatigue, shortness of breath on exertion, and back and rib pain that is made worse with movement. Investigations reveal he is anemic, the calcium, urea, and creatinine are elevated. X-rays reveal multiple lytic lesions in the long bones and ribs, and protein electrophoresis is positive for an immunoglobulin G (IgG) paraprotein. Which of the following is the most likely mechanism for the renal injury?

**Core Concept**
The question is testing the understanding of the pathophysiology of multiple myeloma, a plasma cell malignancy characterized by the proliferation of malignant plasma cells in the bone marrow. The renal injury in multiple myeloma is primarily caused by the deposition of light chains, also known as Bence-Jones proteins, which are produced by the malignant plasma cells.

**Why the Correct Answer is Right**
The correct answer is related to the mechanism of cast nephropathy, which is the most common cause of renal failure in multiple myeloma. The light chains produced by the malignant plasma cells are filtered through the glomeruli and then reabsorbed and catabolized by the proximal tubular cells. However, if the concentration of light chains exceeds the catabolic capacity of the proximal tubules, they precipitate and form casts, which can cause obstruction of the renal tubules and lead to acute kidney injury. This process is known as cast nephropathy.

**Why Each Wrong Option is Incorrect**
**Option A:** Amyloidosis is a condition characterized by the deposition of amyloid fibrils in various tissues, including the kidneys. While amyloidosis can cause renal dysfunction, it is not the primary mechanism of renal injury in multiple myeloma.

**Option B:** Hypercalcemia is a common complication of multiple myeloma, but it is not the direct cause of renal injury. However, hypercalcemia can exacerbate the renal injury caused by cast nephropathy.

**Option C:** Tubulointerstitial nephritis is a condition characterized by inflammation of the renal tubules and interstitium. While it can cause renal dysfunction, it is not the primary mechanism of renal injury in multiple myeloma.

**Option D:** Glomerulonephritis is a condition characterized by inflammation of the glomeruli. While it can cause renal dysfunction, it is not the primary mechanism of renal injury in multiple myeloma.

**Clinical Pearl / High-Yield Fact**
The diagnosis of multiple myeloma is often made by the combination of clinical features, laboratory findings, and imaging studies. The presence of lytic lesions on X-ray, anemia, and elevated calcium and creatinine levels are all suggestive of multiple myeloma. Additionally, the presence of an IgG paraprotein on protein electrophoresis is a key diagnostic finding.

**Correct Answer:** C. Cast nephropathy is the most common cause of renal failure in multiple myeloma, and it is caused by the deposition of light chains in the renal tubules.

✓ Correct Answer: B. tubular damage by light chains