A 30-year-old man with cholera passes 6 liters of watery stool in 12 hours. The stool is isotonic relative to plasma. Assuming he receives no therapy yet, which single statement BEST describes his fluid status?

• A Extracellular fluid volume decreased with unchanged plasma osmolality ✓
• B Intracellular fluid volume decreased with raised plasma osmolality
• C Extracellular fluid volume decreased with lowered plasma osmolality
• D Total body water unchanged with increased extracellular tonicity

Explanation

Isotonic fluid lost externally comes exclusively from the extracellular compartment because only ECF communicates with the gut lumen across capillary membranes. Because the lost fluid carries solute and water in proportional amounts, both ECF volume and its osmolality fall together, leaving no osmotic gradient to shift water out of cells. ICF volume therefore remains essentially constant, unlike the hyponatremia of SIADH or the hypernatremia of pure water loss. This is the classic Darrow-Yannet picture of isotonic volume contraction.

Reference: Guyton and Hall Textbook of Medical Physiology, 14th ed.

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Written and medically reviewed by the StethoPrep medical team.