This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: HAEM synth and porphyrias, Subject name: Biochemistry
[{'content': 'Which vitamin is required for the rate-limiting step of heme synthesis: \nSelect and explain. \nA. Vitamin B1, B. Vitamin B2, C. Vitamin B3, D. Vitamin B6', 'role': 'user'}, {'content': 'Answer: D \nExplanation: The rate-limiting step of heme synthesis is catalysed by ALA synthase This condensation reaction requires Pyridoxal phosphate (Vitamin B6) as a cofactor. This is why vitamin B6 deficiency leads to sideroblastic anemia.', 'role': 'assistant'}]