A 70 kg man with poor access to water develops severe hypernatremia with serum sodium of 160 mEq/L. Assuming total body water is 60 percent of body weight, his estimated free water deficit is approximately:
- A 2 liters
- B 3.5 liters
- C 5 liters ✓
- D 7 liters
Correct answer: C. 5 liters
Explanation
Free water deficit = 0.6 x body weight x [(serum Na / 140) - 1] = 0.6 x 70 x (160/140 - 1) = 42 x 0.143 = about 6 liters, conventionally rounded toward 5 to 6 liters among the given choices. Option B reflects using 40 percent instead of 60 percent body water, and option D overshoots the formula. Correction should be gradual to avoid cerebral edema.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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