This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Medicine
Enzyme defect in acute intermittent porphyria - 
Select and explain. 
A. HMB synthase, B. ALA dehydratase, C. URO I synthase, D. URO II synthase
A: HMB synthase
Answer: A: HMB synthase 
Explanation: None