Review Question - QID 109242

A 7-week-old boy presents to the pediatrician for vomiting. Three weeks ago, the patient began vomiting after meals. The vomitus appears to be normal stomach contents without streaks of red or green. His parents have already tried repositioning him during mealtimes and switching his formula to eliminate cow’s milk and soy. Despite these adjustments, the vomiting has become more frequent and forceful. The patient is voiding about 4 times per day and his urine appears dark yellow. He has fallen 1 standard deviation off of the growth curve. The patient's mother reports that the pregnancy was uncomplicated other than an episode of sinusitis in the 3rd trimester, for which she was treated with azithromycin. The patient's temperature is 98.7°F (37.1°C), blood pressure is 58/41 mmHg, pulse is 166/min, and respirations are 16/min. On physical exam, the patient looks small for his age. His abdomen is soft, non-tender, and non-distended. Which of the following is the most appropriate next step in management?