A 55-year-old man with high-grade non-Hodgkin lymphoma begins chemotherapy. Within 24 hours, he develops muscle cramps, tetany, and a prolonged QT interval on ECG. Which electrolyte abnormality is most likely responsible for these findings?

• A Hyperkalemia
• B Hyperphosphatemia
• C Hyperuricemia
• D Hypocalcemia ✓

Explanation

Hypocalcemia in tumor lysis syndrome results from hyperphosphatemia, where elevated phosphate binds serum calcium. This causes neuromuscular irritability, manifesting as muscle cramps, tetany, and a prolonged QT interval on ECG. Hyperkalemia causes cardiac arrhythmias with peaked T waves, not tetany. Hyperuricemia leads to acute kidney injury but does not directly cause these neuromuscular signs. Hyperphosphatemia drives the hypocalcemia but the clinical symptoms are directly attributable to the low ionized calcium.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

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