Which of the following ions is NOT important in fluid regulation?

Fluid regulation in the body primarily revolves around maintaining the balance of electrolytes, which include sodium, potassium, and chloride. These ions play critical roles in various physiological functions such as osmosis, nerve transmission, and muscle contraction.

Sodium is essential for maintaining extracellular fluid volume and regulating blood pressure, while potassium is crucial for cellular functions, including the regulation of heart rate and muscle contraction. Chloride helps maintain osmotic pressure and is involved in the formation of gastric acid, which is vital for digestion. Additionally, chloride works closely with sodium in maintaining fluid balance.

On the other hand, while sulfur is important for various metabolic processes, including the synthesis of amino acids and detoxification, it does not play a direct role in fluid regulation like the other ions do. Hence, sulfur is not considered a critical ion in the context of regulating body fluids, making it the correct choice in this question.