One nitrogen atom of excreted urea comes directly from free ammonia. What is the source of the second nitrogen atom?

• A Glutamine amide nitrogen transferred by CPS-II
• B Asparagine amide nitrogen released by asparaginase
• C Glutamate alpha-amino group transferred by transaminases
• D Aspartate added by argininosuccinate synthetase ✓

Explanation

In the cytosol, argininosuccinate synthetase condenses citrulline with aspartate, consuming one ATP to AMP. The aspartate amino nitrogen becomes the second urea nitrogen and is released when argininosuccinate lyase cleaves the molecule to arginine and fumarate. Glutamate contributes indirectly because transaminases channel amino groups into oxaloacetate to form this aspartate, but it is not the direct donor. CPS-II uses glutamine but belongs to pyrimidine synthesis, not the urea cycle.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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