A 28-year-old woman presents to the emergency department with acute severe abdominal pain, tachycardia (HR 110/min), and confusion. She has a history of similar episodes triggered by fasting and certain medications. Physical examination reveals dark urine and hypertension (BP 160/95 mmHg). Serum electrolytes show hyponatremia (Na⁺ 128 mEq/L) and hypokalaemia (K⁺ 3.2 mEq/L). Urine porphobilinogen (PBG) is markedly elevated. What is the most likely diagnosis?