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Tapering — MRCP Part 1 MCQ

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HardExercise PhysiologyTaperingMRCP Part 1

A 19-year-old has recurrent confusion after high-protein meals, chronic spasticity and mild intellectual impairment. During an attack ammonia is raised; plasma ornithine is markedly increased and urine contains homocitrulline. Which molecular defect best unifies the findings?

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Correct answer: D — Biallelic SLC25A15 loss impairing mitochondrial ornithine import

The best answer is “Biallelic SLC25A15 loss impairing mitochondrial ornithine import”. The biochemical triad defines hyperornithinaemia–hyperammonaemia–homocitrullinuria syndrome. SLC25A15 encodes the mitochondrial ornithine carrier needed to deliver ornithine for the intramitochondrial urea-cycle reactions. Failure raises cytosolic ornithine, restricts ureagenesis and permits carbamylation of lysine to homocitrulline. Lysinuric protein intolerance lowers plasma cationic amino acids, OTC deficiency does not cause marked hyperornithinaemia, citrin deficiency has a different metabolite pattern, and arginase deficiency raises arginine.

Reference: Hyperornithinaemia–hyperammonaemia–homocitrullinuria syndrome — GeneReviews: https://www.ncbi.nlm.nih.gov/books/NBK97260/