A one-week old boy presents with six episodes of non-bilious vomiting within the last 2 days and reduced urine output for the last 12 hours.

He is also lethargic and is feeding poorly. There is no history of fever or diarrhea.

His antenatal history is uncomplicated, with a birth weight of 3.2 kg. His family history is unremarkable. He was breastfed from birth.