A newborn is diagnosed with citrullinemia type I (deficiency of argininosuccinate synthetase in the urea cycle). Which metabolic consequences are expected?

• A Elevated citrulline, elevated ammonia, low arginine in plasma with orotic aciduria ✓
• B Elevated orotic acid, normal citrulline, hyperammonemia with low argininosuccinate
• C Elevated argininosuccinate, low citrulline, with normal plasma ammonia
• D Elevated arginine, low citrulline, with oculomotor abnormalities

Explanation

In citrullinemia type I (ASS deficiency), citrulline cannot be converted to argininosuccinate, so it accumulates markedly in blood. Ammonia accumulates because the urea cycle is blocked downstream of carbamoyl phosphate and citrulline synthesis. Arginine production is impaired (since arginine is made downstream of the block), so it is low. Orotic aciduria occurs because excess carbamoyl phosphate spills into the pyrimidine synthesis pathway, producing orotic acid. This triad (high citrulline + hyperammonemia + orotic aciduria + low arginine) characterises ASS deficiency.

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

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Written and medically reviewed by the StethoPrep medical team.