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A well 24-year-old woman has routine blood tests before starting a combined oral contraceptive. She takes no medication and her blood pressure is 112/70 mmHg. Blood was taken from a hand vein with a 25 gauge butterfly needle after a three-minute tourniquet, while she clenched her fist repeatedly. Results: potassium 6.4 mmol/L [3.5 to 5.2], sodium 139 mmol/L [135 to 145], bicarbonate 26 mmol/L [22 to 32], creatinine 58 micromol/L [45 to 90], corrected calcium 2.36 mmol/L [2.10 to 2.60], alkaline phosphatase 72 U/L [30 to 110], platelets 268 x 10^9/L [150 to 400]. Which one of the following best explains the potassium result?
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