Hyponatremia due to vomiting and diarrhea is due to which mechanism?

• A. Addison's disease
• B. Prolonged vomiting and diarrhea
• C. Salt losing nephropathy
• D. Ketonuria

Answer: (B)

Loss of sodium and water from the GI tract causes extracellular fluid volume depletion. The body responds by increasing ADH, which promotes water reabsorption in the kidneys. This extra free water dilutes serum sodium, lowering its concentration and creating hyponatremia. Prolonged vomiting and diarrhea are the direct mechanism because they remove Na+ and water, leading to hypo-osmolar, dilutional hyponatremia as water is retained to compensate for volume loss. Other options involve different pathways—hormonal deficiencies affecting sodium balance, renal sodium wasting, or ketosis-related changes—that don’t explain the hyponatremia produced by GI losses.