During the diuretic phase on days 2–3 of life, which parameter shows a transient rise?

• A. Urine output
• B. Creatinine
• C. Blood pressure
• D. FENA

Answer: (D)

During the neonatal transition, the diuretic phase around days 2–3 involves a temporary natriuresis as the kidneys shift from placental fluid clearance to urine production. The tubular handling of sodium is immature, so the tubules excrete more sodium relative to what is filtered. This causes the fractional excretion of sodium to rise transiently. FENA reflects this tubular sodium loss and is the most specific indicator of this temporary natriuresis. Urine output does increase during this phase, but the key distinctive change here is the temporary rise in how much sodium is being excreted relative to filtration, which FENA captures. As renal maturation progresses, FENA returns toward baseline.