Hyponatremia is defined as:

• A. High sodium content of blood
• B. Normal sodium content
• C. Elevated sodium in tissues
• D. Low sodium content of blood and tissues

Answer: (D)

Hyponatremia means there is too little sodium in the circulating fluid. The defining measure is a low serum (blood) sodium concentration, which reflects a reduced extracellular sodium amount and can cause water to shift into cells. The option that describes low sodium content of blood and tissues best aligns with this systemic low-sodium state (clinical focus is on serum sodium, typically below 135 mEq/L). The other choices describe high sodium, normal sodium, or elevated sodium in tissues, which do not define hyponatremia.