Final answer:
Someone exercising vigorously is at the greatest risk of developing water toxicity because they may consume large amounts of water and not replenish electrolytes, leading to a dilution of electrolytes in the bloodstream and water intoxication.
Step-by-step explanation:
People at the greatest risk of developing water toxicity are those who are likely to experience conditions that lead to excessive water intake or those who have health problems affecting water balance. Among the options provided, someone who is exercising vigorously is particularly at risk as they may consume a lot of water to prevent dehydration, especially in hot weather or during long bouts of physical activity. They may not replenish electrolytes lost through sweat, leading to a dilution of electrolytes in their bloodstream, causing water intoxication. Factors such as diabetes mellitus or diabetes insipidus, excess caffeine or alcohol, kidney disease, certain drugs like diuretics, and conditions like sickle cell anemia can also contribute to the risk, along with an excessive water intake.