Final answer:
The symptoms align with anticholinergic and cardiotoxic effects, potentially from tricyclic antidepressant (TCA) overdose. A toxicology screen is necessary for definitive identification.
Step-by-step explanation:
The presentation of dry mouth, tachycardia, vomiting, urinary retention, seizures, and changes in the electrocardiogram with a widened QRS and prolonged QT intervals suggests that the pediatric patient ingested a medication with anticholinergic and cardiotoxic properties. The history of epilepsy might be relevant if the patient was on medication for this condition, which could have narrow therapeutic windows and are prone to causing toxicity when overdosed.
The symptoms do not align exactly with the expected adverse effects of cold syrup overdose, which more commonly includes hallucinations and involuntary movements, nor with phenobarbital, which tends to cause central nervous system depression rather than seizures. Considering the EKG findings, a possible cause could be an overdose of tricyclic antidepressants (TCAs), which are known to have a substantial impact on the cardiac conduction system and can lead to the observed symptoms. However, without knowing the specific pills ingested, this remains a differential diagnosis, and a toxicology screen would be crucial for final confirmation.