HHH syndrome is due to a defect of which pathway?

Ans. is 'd' i.e., Urea cycle* In urea cycle, as the enzyme ornithine transcarbamylase is present in mitochondria, ornithine should be taken into mitochondria. Ornithine then reacts with carbamyl phosphate to form citrulline within mitochondria. After citrulline is formed, it should leave the mitochondria to reach cytoplasm.* Hence ornithine should get into mitochondria and citrulline should leave the mitochondria. This is by ornithine citrulline transporter.* Defect of ornithine citrulline transporter results in HHH syndrome characterised by# Hyperammonemia (as ornithine can not get into mitochondria, urea cycle is inhibited and hence ammonia accumulates)# Hyperornithinemia (as ornithine is not being utilised for urea cycle)# Hyperhomocitrullinemia (when carbamyl phosphate of urea cycle does not find ornithine within mitochondria, it reacts with lysine to form homocitrulline and hence homocitrullinemia)* Thus HHH syndrome or Hyperammoniemia Hyperornithinemia Hyperhomocitrullinemia is caused by Ornithine Citrulline transporter present in mitochondrial membrane.