Final answer:
Infants and the elderly are the most vulnerable to dehydration due to infants' low body mass and the elderly's less sensitive thirst mechanism, which can lead to chronic dehydration.
Step-by-step explanation:
The groups that are the most vulnerable to dehydration are infants and the elderly. Infants are at high risk because of their low body mass and because they can become dangerously dehydrated very quickly, especially if they experience frequent vomiting or diarrhea. The elderly are also at significant risk since their thirst mechanism may not be as sensitive, making them susceptible to chronic dehydration and its associated health risks such as kidney disease and heart disease. Additionally, endurance athletes, including distance runners who perform long races, can become dehydrated if they do not consume adequate fluids. It is crucial for these vulnerable groups to maintain proper hydration for their health and well-being.