This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Urea cycle, Subject name: Biochemistry
HHH syndrome is due to defect in:- 
Select and explain. 
A. Ornithine transcarbamoylase, B. Ornithine permease, C. Ornithine decarboxylase, D. Ornithine
B: Ornithine permease
Answer: B: Ornithine permease 
Explanation: HHH syndrome is due to defect in Ornithine permease of mitochondrial membrane. Hyperornithinemia: As ornithine can't enter mitochondria, it accumulates in blood. Hyperammonemia: As ornithine is not available, urea cycle can't operate i.e. NH3 can't be detoxified. Homocitrullinuria: As ornithine is not available, ornithine homologue lysine reacts with Carbamoyl phosphate to produce homocitrulline. (Difference between lysine and ornithine is just one CH2 group)