Infantile hypertrophic pyloric stenosis — preoperative stabilization

A 2-week-old male infant born at 38 weeks gestation presents with projectile vomiting 30 minutes after feeding, followed by hungry feeding immediately after. On examination, visible peristaltic waves are noted moving from left to right across the epigastrium. The infant appears dehydrated with sunken fontanelle. Serum electrolytes show pH 7.48, HCO3− 32 mEq/L, K+ 3.0 mEq/L, and Cl− 88 mEq/L. Abdominal ultrasound reveals a pyloric muscle thickness of 4.5 mm and pyloric channel length of 16 mm. Which of the following is the MOST appropriate next step in management?