In an infant with prerenal oliguria without renal failure, urine osmolarity is typically:

• A. Increased
• B. Decreased
• C. Within Normal Limits
• D. Variable

Answer: (A)

In prerenal oliguria, reduced renal perfusion triggers the kidneys to conserve water and sodium to preserve intravascular volume. The tubules stay intact, so they can concentrate urine effectively. This heightened concentrating ability leads to an increased urine osmolarity, often well above plasma levels (commonly over about 500 mOsm/kg, though exact numbers can vary). This pattern helps distinguish prerenal states from intrinsic renal injury, where the tubules can’t concentrate urine as well and osmolarity tends to be lower or inappropriately normal. So the typical finding in this scenario is increased urine osmolarity.