A 28-year-old woman from rural Maharashtra presents to the emergency department with acute onset severe abdominal pain, tachycardia (HR 110/min), and confusion. She reports a 3-day history of constipation and dark urine. On examination, she has proximal muscle weakness and hyponatraemia (Na+ 128 mEq/L). Urine porphobilinogen (PBG) is markedly elevated at 50 mg/L (normal <4 mg/L). What is the most appropriate immediate management?