A 29-year-old man presents after completing an unusually strenuous cross-country event with severe thigh pain and dark urine. His creatinine concentration is 286 micromol/L (60–110), creatine kinase is 38 000 U/L (40–250), and urine dipstick testing is strongly positive for blood, although microscopy shows no red cells. His haemoglobin, platelet count and lactate dehydrogenase are normal. Which one of the following is the most likely cause of his acute kidney injury?