This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Kidney, Subject name: Medicine
A 74-year-old man presents with fatigue, shortness of breath on exertion, and back and rib pain, which is made worse with movement. Investigations reveal he is anemic, 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? 
Select and explain. 
A. plasma cell infiltrates, B. tubular damage by light chains, C. glomerular injury, D. vascular injury by light chains
B: tubular damage by light chains
Answer: B: tubular damage by light chains 
Explanation: In multiple myeloma, tubular damage by light chains is almost always present. The injury is a direct toxic effect of the light chains or indirectly from the inflammatory response. Infiltration by plasma cells and glomerular injury is rare. Hypercalcemia may produce transient or irreversible renal damage as do amyloid and myeloma cell infiltrates.