Two hours after starting chemotherapy for acute lymphoblastic leukaemia, a child develops severe hyperkalaemia, hyperphosphataemia, and hyperuricaemia. Serum calcium falls from 9.4 to 5.9 mg/dL. Which mechanism best explains the fall in calcium?
- A Released tumour cell PTHrP acting on osteoblasts
- B Suppression of renal 1-alpha hydroxylase by released cytokines
- C Elevated phosphate complexing with calcium and precipitating in soft tissues ✓
- D Direct cytotoxic effect of cisplatin on parathyroid chief cells
Correct answer: C. Elevated phosphate complexing with calcium and precipitating in soft tissues
Explanation
Tumour lysis syndrome releases large quantities of intracellular phosphate when nucleated cells are destroyed. The solubility product of calcium and phosphate is exceeded, so calcium phosphate precipitates in soft tissues including renal tubules, lowering ionised calcium. Hyperphosphataemia is therefore the direct driver of hypocalcaemia here. PTHrP mediates humoral hypercalcaemia of malignancy rather than lysis-induced hypocalcaemia, and alkalinisation of urine must be avoided precisely because it promotes this precipitation.
Reference: Nelson Textbook of Pediatrics, 22nd ed.
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Written and medically reviewed by the StethoPrep medical team.