Dehydration due to inadequate breast milk intake: The infant is having only a few drops of colostrum, which may not be sufficient for adequate hydration, especially if the mother's milk has not yet come in. This is a common issue in the first few days of life, especially if the mother is a first-time breast-feeder or if there are any issues with latching or milk letdown.

Normal variation in urine output: Newborns, especially those who are breast-fed, may have varying patterns of urine output, and it's not uncommon for them to have a dry period after the initial meconium and urine-filled diaper.
Delayed maturation of the kidneys: Newborn kidneys are immature, and it may take some time for them to fully mature and produce adequate amounts of urine.

Congenital urinary tract anomalies: Although unlikely, it's essential to consider congenital urinary tract anomalies, such as posterior urethral valves or renal agenesis, which can cause oliguria or anuria.
Severe dehydration due to other causes: Other causes of severe dehydration, such as gastrointestinal obstruction or congenital heart disease, must be considered, although they are less likely given the infant's otherwise healthy and vigorous appearance.

Congenital adrenal hyperplasia: This rare condition can cause dehydration and electrolyte imbalances due to adrenal insufficiency.
Nephrogenic diabetes insipidus: A rare condition characterized by the kidneys' inability to respond to antidiuretic hormone, leading to excessive urine production, although this would typically present with polyuria rather than oliguria.