A 20 kg child has pure water deprivation hypernatremia with serum sodium 160 mEq/L and no other electrolyte abnormalities. Approximately what free water deficit must be replaced?
- A About 850 mL
- B About 1200 mL
- C About 1700 mL ✓
- D About 3200 mL
Correct answer: C. About 1700 mL
Explanation
Free water deficit equals 0.6 multiplied by body weight multiplied by the fraction (measured sodium divided by 140) minus one. For this child: 0.6 times 20 kg gives total body water of 12 L, and 160 divided by 140 minus 1 equals approximately 0.143, so the deficit is about 1.7 L. The figure of 3200 mL double counts the entire extracellular tonicity error, while 850 and 1200 mL underestimate the deficit and would leave clinically significant hypertonicity uncorrected.
Reference: Nelson Textbook of Pediatrics, 21st ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.