A neonate presents with lethargy, poor feeding, and elevated ammonia levels. Urine analysis reveals elevated orotic acid. A deficiency in which enzyme is the most likely cause?

✓ Correct Answer: Option B

Ornithine transcarbamylase (OTC) deficiency is an X-linked urea cycle disorder. The enzyme defect leads to accumulation of carbamoyl phosphate, which is then shunted to pyrimidine synthesis, resulting in elevated orotic acid and hyperammonemia.