In ornithine transcarbamylase (OTC) deficiency, the most common urea cycle disorder, which amino acid becomes conditionally essential and why?

• A Phenylalanine, because the aromatic amino acid pathway is blocked
• B Lysine, because ornithine and lysine share transporters
• C Glutamine, because it is consumed as a nitrogen donor
• D Arginine, because the urea cycle cannot complete the citrulline → argininosuccinate → arginine → urea → ornithine cycle, depleting arginine ✓

Explanation

OTC deficiency blocks the condensation of carbamoyl phosphate with ornithine in the mitochondria, halting the cycle. Arginine is normally regenerated in the cycle (argininosuccinate → arginine via ASL). When the cycle is blocked, arginine cannot be synthesized endogenously; all dietary and endogenous protein breakdown of arginine goes to catabolism, depleting it. Arginine becomes an essential amino acid in OTC and other proximal urea cycle defects. Supplemental arginine (with sodium benzoate/phenylbutyrate) is given to provide the alternative pathway nitrogen-scavenging and to supplement arginine.

Reference: Harper's Illustrated Biochemistry, 32nd ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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