A 35-year-old man from Delhi presents with recurrent episodes of photosensitive blistering lesions on the dorsal surfaces of his hands and face, triggered by sun exposure. He also reports dark urine and mild hepatic dysfunction (AST 2× ULN). Urine uroporphyrin is elevated at 800 μmol/24 h (normal <100), and faecal coproporphyrin is elevated. Plasma porphyrins are normal. What is the most appropriate next step in management?