A client is hospitalized for observation after suffering heat exhaustion during a marathon race. He complains of dizziness when he stands up,and the RN determines that he has postural hypotension. Based on the information which of the following electrolyte imbalances could cause the client's symptoms?

The correct answer is B: Hyponatremia. Hyponatremia, or low sodium levels, can lead to dizziness, especially upon standing up, due to its effect on blood volume and blood pressure regulation. Sodium plays a crucial role in maintaining the body's fluid balance, and low levels can result in decreased blood volume and subsequent postural hypotension. Other choices, such as hypernatremia (A), hyperkalemia (C), and hypokalemia (D), do not directly affect blood pressure regulation in the same way that hyponatremia does. Hypernatremia and hyperkalemia can actually lead to high blood pressure, while hypokalemia can cause muscle weakness and cardiac arrhythmias but not necessarily dizziness upon standing up. Therefore, hyponatremia is the most likely electrolyte imbalance to cause the client's symptoms in this scenario.