A neonate with a urea cycle defect has a plasma urea of 5 mg/dL (markedly low). This would have which effect on the kidney's maximum urine concentrating ability?
- A Maximum urine osmolality decreases because urea contributes about half the inner medullary osmotic gradient ✓
- B Maximum urine osmolality increases because urea is an osmotic diuretic
- C No change because NaCl alone maintains the medullary gradient
- D Maximum urine osmolality decreases because urea is required for NKCC2 function
Correct answer: A. Maximum urine osmolality decreases because urea contributes about half the inner medullary osmotic gradient
Explanation
Urea contributes roughly half of the inner medullary interstitial osmolality (600 of 1200 mOsm/kg). Urea recycling via UT-B1 and UT-B3 in the inner medullary collecting duct is essential for maximum concentrating ability. Low medullary urea reduces maximum urine osmolality. NaCl alone can only achieve about 600 mOsm/kg in the inner medulla.
Reference: BRS Physiology, 7th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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