This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Metabolism of Purines and Pyrimidines, Subject name: Biochemistry
Rate limiting step in porphyrine synthesis - 
Select and explain. 
A. ALA dehydratase, B. ALA synthase, C. UPG decarboxylase, D. Ferrochelatase
B: ALA synthase
Answer: B: ALA synthase 
Explanation: Ans. is 'b' i.e., ALA synthase o See above explanation. Heme is the most important porphyrin (porphyrin III) in the body.