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: Biochemistry
HHH syndrome is due to defect in ? 
Select and explain. 
A. Tryptophan metabolism, B. Histidine transpoer, C. Branched chain AA metabolism, D. Ornithine transpoer
D: Ornithine transpoer
Answer: D: Ornithine transpoer 
Explanation: Ans. is 'd' i.e., Ornithine transpoer Hyperornithinaemia, hyperammonaemia, homocitrullinuria (HHH) syndrome is an autosomal recessive disorder of ornithine transpo caused by mutations in gene SLC 25A15 encoding the ornithine transoer protein (ORNT1).There is defective activity of the ornithine transpoer across the mitochondrial membrane, which causes a functional deficiency of two mitochondrial enzymes:Ornithine transcarbomylase : Which catalyses the condensation of ornithine and carbamoylphosphate to citrulline.Ornithine-8-aminotransferase (OAT) : Which metabolizes the ornithine to .'-pyrroline-5-carboxylate and ultimately glutamate and proline.Ornithine accumulates in the cytoplasm and its deficiency in mitochondria causes a secondary urea cycle disorder and hyperammonemia.Carbamoylphosphate accumulates and undergoes allternate metabolism to form :Homocitrulline - Excreted in urineOrotic acidPlasma