A patient with hereditary hyperammonemia due to a disorder of ornithine cycle has developed secondary orotaciduria. The increased synthesis of orotic acid is caused by an increase in the following metabolite of ornithine cycle:

Answer options

1. Argininosuccinate
2. Urea
3. Carbamoyl phosphate
4. Citrulline
5. Ornithine

Explanation

Lack of ornithine carbamoyltransferase that is supposed to covert carbamoyl + ornithine to citrulline gives room for the build-up of carbamoyl phosphate which is a substrate for the orotic cycle. ( Orotic cycle;- carbamoyl phosphate + aspartate is converted to N- carbamoyl aspartate (by ASPARTATE TRANSCARBAMYLASE) which is converted to dihydroorotate (by dihydroorotase) which is converted finally to orotic acid (by dihydroorotate dehydrogenase). Simply put, carbamoyl phosphate is required in both cycles.