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
Rate limiting enzyme of Haem synthesis in erythroid tissue is: 
Select and explain. 
A. PBG deaminase, B. Uroporphyrin II cosynthetase, C. ALA synthase I, D. ALA synthase II
D: ALA synthase II
Answer: D: ALA synthase II 
Explanation: RLE in Haem Synthesis is - ALA Synthase ALA Synthase I : Expressed throughout the body. (Rate limiting step in liver) ALA Synthase II: Expressed in erythroid tissues