A 35-year-old Indian man presents with a 2-year history of progressive photosensitivity, blistering on the dorsal surfaces of hands and face, and hyperpigmentation. He has a history of hepatitis C infection and mild hepatic dysfunction. Serum transaminases are mildly elevated. Urine shows increased uroporphyrin (predominantly Type III isomer). Faecal porphyrins are also elevated. Which enzyme deficiency is responsible for this condition?