Which of the following is a key sign of hypokalemia?

Muscle weakness is a key sign of hypokalemia due to the essential role that potassium plays in muscle function. Potassium is necessary for maintaining proper electrical conduction in muscles and in the nervous system. When potassium levels drop below normal, it can lead to reduced muscle strength and increased muscle fatigue, which can manifest as weakness. This weakness can affect various muscle groups, and in severe cases, it may even impact respiratory muscles, making it critical to recognize.

While the other options may be associated with different medical conditions or electrolyte imbalances, they do not specifically characterize hypokalemia. Fluid retention is more commonly associated with conditions like heart failure or nephrotic syndrome. Confusion can arise from various causes, including metabolic disturbances, but it is not a hallmark sign of low potassium levels. Severe dehydration often involves other electrolyte imbalances and is not specific to hypokalemia alone. Therefore, muscle weakness directly correlates with the effects of inadequate potassium in the body, making it the correct answer.