Final answer:
The antimanic agent likely responsible for the described symptoms of a 24-year-old patient with a mood disorder who has carnitine metabolism disorder is valproate. This medication can deplete carnitine, leading to symptoms like confusion, irritability, and an increased risk of seizures.
Step-by-step explanation:
The patient described in the question is experiencing confusion, irritability, and seizures after starting on an antimanic agent, and they have a disorder of carnitine metabolism. Based on these symptoms, the antimanic agent that could be responsible is valproate (b). Valproate is a medication that has been associated with the depletion of carnitine and can lead to hyperammonemia, which might explain the confusion and irritability. Moreover, it can increase seizure risk in individuals with underlying metabolic disorders, particularly those involving carnitine metabolism.