Conversion of the linear tetrapyrrole hydroxymethylbilane to uroporphyrinogen III (A) Occurs spontaneously (B) Catalysed by uroporphyrinogen I synthase (C) Catalysed by uroporphyrinogen III cosynthase (D) Catalysed by combined action of uroporphyrinogen I synthase and uroporphyrinogen III cosynthase