A marathon runner presents confused and has a seizure after drinking excessive plain water during the race. Which of the following analyses best identifies the most likely primary diagnosis and immediate pathophysiologic cause?

• A. Hypoglycemic seizure due to inadequate carbohydrate intake during endurance exercise rather than any fluid or electrolyte disturbance, which is the most common cause of seizures in marathon runners
• B. Heat stroke from hyperthermia causing breakdown of the blood–brain barrier independent of sodium changes and leading to seizures because plasma sodium is typically elevated in this condition
• C. Exercise-associated hyponatremia due to ingestion of excessive hypotonic fluids diluting ECF sodium, causing water to move into brain cells and resulting in cerebral edema and neurologic symptoms
• D. Severe dehydration causing hypernatremia and intracellular dehydration of neurons, with seizures due to neuronal shrinkage rather than swelling

Answer: (C)