A 30-year-old man is brought after a crush injury from a building collapse. He has swollen, tense thighs. Creatinine kinase is 48,000 U/L. Serum creatinine is 4.2 mg/dL (baseline 0.9). Serum calcium is 7.1 mg/dL, potassium 6.1 mg/dL, phosphate 5.9 mg/dL. Which is the MOST likely explanation for the hypocalcaemia in this setting?

• A PTH resistance from hyperphosphataemia
• B Secondary hyperparathyroidism
• C Reduced 1-alpha hydroxylase activity
• D Calcium deposition in damaged soft tissues ✓

Explanation

In rhabdomyolysis, early hypocalcaemia results from calcium phosphate precipitation in damaged muscle and soft tissues. This is not due to PTH deficiency. Distractor C (reduced 1-alpha hydroxylizer) causes hypocalcaemia in chronic CKD but not acutely. Distractor A is circular: hyperphosphataemia does not cause PTH resistance. Hypercalcaemia may occur later in the recovery phase as calcium is mobilized from deposits.

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

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Written and medically reviewed by the StethoPrep medical team.