Which electrolyte abnormality is characteristic of tumor lysis syndrome?

• A. Hyperkalemia
• B. Hypercalcemia
• C. Hyponatremia
• D. Hypophosphatemia

Answer: (A)

When tumor cells break open rapidly, their intracellular contents spill into the bloodstream, triggering a surge of potassium first. That release makes hyperkalemia the most characteristic and immediate electrolyte change in tumor lysis syndrome. Alongside potassium, phosphate also floods the blood (hyperphosphatemia), calcium binds this phosphate and drops (hypocalcemia), and nucleic acids raise uric acid levels (risking uric acid nephropathy). Hyponatremia isn’t a defining TLS feature, and calcium tends to be low rather than high, so hypercalcemia isn’t expected. Hypophosphatemia would contradict the expected rise in phosphate from cell contents. So the electrolyte abnormality that best fits TLS is hyperkalemia.