What would be the strongest evidence that a child has ingested drain cleaner?

The presence of burns to the mouth is the strongest evidence of drain cleaner ingestion. Drain cleaners often contain highly caustic substances, such as sodium hydroxide or sulfuric acid, which can cause severe chemical burns upon contact with mucous membranes and soft tissues. Observing visible burns or damage in the oral cavity can indicate that the corrosive nature of the substance has had a direct and damaging effect, strongly suggesting that the child has ingested the toxic product.

Other signs, such as elevated heart rate, garbled speech, or persistent crying, can be consequences of distress or pain, but they are not definitive indicators of specific poison ingestion. These symptoms can arise from a multitude of other medical conditions and do not provide conclusive evidence of drain cleaner ingestion. Therefore, the presence of burns in the mouth directly points to chemical causticity and the likelihood of ingestion, making it the most compelling evidence in this scenario.