A 32-week preterm neonate fed a concentrated formula develops fever, weight loss and a serum sodium of 158 mEq/L within days. The renal limitation that predisposes such infants to this complication is:

• A Inability to dilute urine below 100 mOsm/kg because of absent ADH receptors
• B High glomerular filtration rate exceeding tubular reabsorptive capacity for water
• C Excessive sodium excretion caused by aldosterone resistance in the distal tubule
• D Limited maximum urinary concentrating ability of about 600 mOsm/kg due to short loops of Henle and a low medullary urea gradient ✓

Explanation

The neonatal kidney can concentrate urine only to about 600 mOsm/kg, half the adult maximum of around 1200, because the loops of Henle are short and the inner medullary interstitium has little accumulated urea. Preterm infants are even more limited, so a solute-rich feed obligates large water losses and produces hypernatremic dehydration. Diluting ability and ADH responsiveness are intact, and neonatal GFR is low, not high, ruling out the remaining options.

Reference: Nelson Textbook of Pediatrics, 21st ed.

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Written and medically reviewed by the StethoPrep medical team.