Final answer:
Hyponatremia is a medical condition that can occur in ultra-endurance events when blood levels of sodium become critically low due to excess water accumulation in the body. It can result from an imbalance of sodium or dilution of sodium due to water retention.
Step-by-step explanation:
Hyponatremia is the serious medical condition that can occur in ultra-endurance events when water alone is used to replace fluid loss and blood levels of sodium become critically low.
Hyponatremia is a lower-than-normal concentration of sodium, usually associated with excess water accumulation in the body, which dilutes the sodium.
A relative decrease in blood sodium can occur because of an imbalance of sodium in one of the body's other fluid compartments, or from a dilution of sodium due to water retention related to edema or congestive heart failure.