A child has recurrent episodes of confusion with hyperammonemia. Plasma ornithine is persistently elevated, citrulline is low-normal, and urinary homocitrulline is present. Liver enzymes are normal. The defect lies in:

• A Cytosolic arginase
• B The mitochondrial inner membrane ornithine transporter (ORNT1, SLC25A15) ✓
• C Carbamoyl phosphate synthetase-I
• D The plasma membrane dibasic amino acid transporter y+ (SLC7A7)

Explanation

Hyperornithinemia-hyperammonemia-homocitrullinuria (HHH) syndrome results from ORNT1 deficiency, impairing ornithine entry into mitochondria. Ornithine accumulates in plasma while intramitochondrial ornithine shortage limits OTC activity, diverting carbamoyl phosphate toward lysine to form homocitrulline. Option D describes lysinuric protein intolerance, which features low plasma lysine and arginine with pulmonary and immune complications. Arginase deficiency raises arginine, and CPS-I deficiency gives low citrulline without hyperornithinemia or homocitrulline.

Reference: Nelson Textbook of Pediatrics, 21st ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.