Pyloric Stenosis — Visible Peristalsis

A 4-week-old first-born male infant is brought to the pediatric emergency department with a 10-day history of projectile non-bilious vomiting after feeds. The mother reports the infant appears hungry immediately after vomiting. On examination, the structure marked **A** in the diagram—visible gastric peristalsis moving left-to-right across the epigastrium—is clearly observed. Laboratory investigations reveal serum pH 7.52, HCO₃⁻ 32 mEq/L, K⁺ 2.8 mEq/L, and Cl⁻ 88 mEq/L. Urine pH is 5.2 despite systemic alkalosis. What is the MOST appropriate immediate management before surgical intervention?