Physiologycommon
A 34-year-old man is seen in the medical tent 20 minutes after finishing a marathon run on a cool, overcast morning. He drank water at every drink station. He has a headache and painful cramps in both calves, and has been intermittently confused for the past hour. His blood pressure is 128/76 mmHg. He was weighed at registration and now weighs 2.4 kg more than he did then. Plasma sodium is 122 mmol/L.
Which one of the following findings favours excess water intake rather than sodium lost in sweat as the cause of this man's low plasma sodium concentration?