Q. A 3-year-old boy is brought to the pediatric clinic by his parents due to their concern about his growth and development. They report that he seems smaller than his peers and has not been reaching developmental milestones at the expected pace. On examination, the boy has noticeable spasticity in his lower extremities. His medical history reveals episodes of unexplained irritability and vomiting, often triggered by illnesses like colds or flu. Blood tests reveal increased serum arginine and hyperammonemia, along with decreased blood urea nitrogen (BUN). Given the clinical presentation and laboratory findings, which of the following is the most likely diagnosis?