A 7-month-old presents with recurrent episodes of vomiting, lethargy, and confusion after weaning onto solid protein foods. Investigations during an episode show hyperammonemia, elevated plasma ornithine, and urinary homocitrulline. Liver synthetic function is normal. Which transport defect is responsible?

• A Basolateral dibasic amino acid transporter (SLC7A7)
• B Mitochondrial ornithine transporter (ORNT1, SLC25A15) ✓
• C Neutral amino acid transporter of intestine and kidney
• D Cystine-glutamate antiporter of renal tubule

Explanation

Hyperornithinemia-hyperammonemia-homocitrullinuria (HHH) syndrome results from ORNT1 deficiency, blocking ornithine entry into mitochondria. Intramitochondrial ornithine depletion stalls the urea cycle causing postprandial hyperammonemia, while carbamoyl phosphate spills into pyrimidine synthesis forming homocitrulline. SLC7B7 defect causes lysinuric protein intolerance, which features low plasma dibasic amino acids rather than high ornithine.

Reference: Nelson Textbook of Pediatrics, 21st ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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