A 4-week-old first-born male presents with projectile non-bilious vomiting after feeds for 1 week. He remains hungry after vomiting but shows signs of dehydration and poor weight gain. On examination, an olive-sized mass is palpable in the epigastrium. Ultrasound confirms hypertrophic pyloric stenosis. Laboratory investigations reveal the electrolyte abnormality marked as **D** in the diagram. Which of the following is the MOST CRITICAL reason why this electrolyte derangement must be corrected BEFORE surgical intervention?