Hyperphosphatemia in renal failure is explained by which mechanism?

• A. Inability to excrete phosphate
• B. Crush injury
• C. Massive cellular lysis releasing phosphate
• D. Increased calcium citrate complexation

Answer: (B)

In renal failure, hyperphosphatemia mainly happens because the kidneys lose the ability to excrete phosphate as the glomerular filtration rate falls. Phosphate is normally filtered by the glomeruli and largely eliminated via the urine; with fewer functioning nephrons, phosphate clearance drops and serum phosphate builds up. While massive cellular breakdown (crush injuries or widespread lysis) can raise phosphate by releasing it from cells, that mechanism is not what explains the chronic rise in phosphate seen with reduced kidney function. Increased calcium citrate complexation isn’t the driver of elevated phosphate in this context, as it alters calcium handling rather than causing renal phosphate retention.