Which statement best describes the renal handling of a water load in a healthy full-term neonate during the first week of life?
- A Maximum urine concentrating capacity approaches adult values because of high medullary tonicity
- B Glomerular filtration exceeds tubular reabsorptive capacity, producing obligatory polyuria
- C Maximum urine concentrating capacity is limited to roughly half of adult values because of a short loop of Henle and low urea supply ✓
- D Antidiuretic hormone secretion is absent until the hypothalamo-neurohypophyseal axis matures at 6 months
Correct answer: C. Maximum urine concentrating capacity is limited to roughly half of adult values because of a short loop of Henle and low urea supply
Explanation
The neonatal kidney can concentrate urine only to about 600 to 700 mOsm/kg, roughly half the adult maximum of about 1200 mOsm/kg, because the loops of Henle are short, medullary interstitial tonicity is low, and urea delivery to the inner medulla is limited. ADH secretion is present at birth, so the defect is a limited concentrating gradient, not absence of the hormone. This immaturity underlies the vulnerability of neonates to dehydration.
Reference: Nelson Textbook of Pediatrics, 21st ed.
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