Review Question - QID 212856

A 10-day-old male infant is brought to the emergency room for abdominal distension for the past day. He has been refusing feeds for about 1 day and appears more lethargic than usual. While changing his diaper today, she noticed that the baby felt warm. He has 1-2 wet diapers a day and has 1-2 seedy stools a day. The mother reports an uncomplicated vaginal delivery. His medical history is significant for moderate respiratory distress following birth that has since resolved. His temperature is 101°F (38.3°C), blood pressure is 98/69 mmHg, pulse is 174/min, respirations are 47/min, and oxygen saturation is 99% on room air. A physical examination demonstrates a baby in moderate distress with abdominal distension. Bedside ultrasound is notable for a distended bladder. Which of the following is the most appropriate initial step in management?