**Question:** A woman runs a marathon in 90degF weather and replaces all volume lost in sweat by drinking distilled water. After the marathon, she will have

A. Hyponatremia
B. Hypernatremia
C. Normonatremia
D. Hypokalemia

**Correct Answer:** A. Hyponatremia

**Core Concept:**
In situations of excessive sweating and inadequate fluid replacement, the body tries to maintain osmotic balance by diluting blood with excess water intake.

**Why the Correct Answer is Right:**
When the woman drinks distilled water, which is devoid of electrolytes, she does not replenish the lost sodium ions (Na+) in her sweat. As a result, the sodium concentration in her blood (serum sodium) decreases, leading to hyponatremia.

**Why Each Wrong Option is Incorrect:**
A. Hypernatremia is the opposite of hyponatremia and occurs when there is excessive loss of Na+ in the body and inadequate intake of water. In this scenario, the woman is replacing her lost volume, not maintaining or increasing Na+ levels.
C. Normonatremia is the normal range of serum sodium concentration. In this case, the woman's serum sodium is not within this range.
D. Hypokalemia occurs when there is loss of potassium (K+) ions in the body and inadequate intake of potassium-rich fluids. In this scenario, the woman is replacing volume and water, not potassium.

**Clinical Pearl:**
Ensuring adequate electrolyte replacement is crucial when rehydrating after physical exertion to prevent hyponatremia and other electrolyte imbalances. Drinking water without electrolytes can lead to potentially life-threatening conditions like brain herniation due to excessive fluid buildup in the brain cells.