The urea cycle converts toxic ammonia into urea for renal excretion. Which enzyme deficiency in the urea cycle results in orotic aciduria along with hyperammonemia?

• A Ornithine transcarbamylase (OTC) deficiency ✓
• B Carbamoyl phosphate synthetase I (CPS-I) deficiency
• C Argininosuccinate synthetase deficiency (citrullinemia)
• D Arginase deficiency

Explanation

OTC deficiency is the most common urea cycle disorder and is X-linked. When OTC is deficient, carbamoyl phosphate accumulates in mitochondria and spills into the cytosol, where it enters the pyrimidine synthesis pathway via aspartate transcarbamylase, generating excess orotic acid excreted in urine. CPS-I deficiency also causes hyperammonemia but lacks orotic aciduria because carbamoyl phosphate does not reach cytosolic pyrimidine synthesis.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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