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
Which vitamin is required for the rate-limiting step of heme synthesis: 
Select and explain. 
A. Vitamin B1, B. Vitamin B2, C. Vitamin B3, D. Vitamin B6
D: Vitamin B6
Answer: D: Vitamin B6 
Explanation: 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.