A patient with syndrome of inappropriate ADH secretion (SIADH) has serum Na 118 mEq/L with normal body weight. Which body fluid compartment change BEST characterizes SIADH?

• A Decreased ECF volume and increased ICF volume with total body sodium deficit
• B Expanded ECF volume with normal to elevated total body sodium and dilutional hyponatremia
• C Expanded ECF and ICF volumes with normal total body sodium and dilutional hyponatremia ✓
• D Equal expansion of ECF and ICF volumes with increased total body sodium

Explanation

In SIADH, inappropriately high ADH causes water retention without sodium retention. Water is distributed across all compartments proportional to osmolarity; both ECF and ICF expand. Serum sodium falls (dilutional hyponatremia) because water is added to both compartments. Total body sodium is normal or mildly elevated (patients are often volume-expanded enough to natriurese excess sodium). Option A describes hypovolemic hyponatremia; option B is close but doesn't specify both compartments correctly; option D implies sodium retention which is not characteristic.

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

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