Hyponatremia is defined as:

• A. High sodium content in blood
• B. Sodium content unchanged
• C. Elevated sodium in tissues
• D. Low sodium content in blood and tissues

Answer: (D)

Hyponatremia is defined by a low concentration of sodium in the extracellular fluid, most notably in the blood. Sodium governs the osmotic balance outside cells, so when serum sodium drops, water shifts into cells and can cause symptoms, especially in the brain. The option describing low sodium content in blood and tissues aligns with this idea, since the key feature is reduced sodium in the extracellular compartment rather than an increase in sodium. The other statements describe high, unchanged, or localized tissue sodium, which don’t fit the definition of hyponatremia.