Which of the following is NOT a clinical sign of metaldehyde toxicity?

Metaldehyde toxicity is typically associated with several clinical signs that result from its effects on the central nervous system and metabolic processes. Common manifestations include hyperthermia due to increased metabolic activity, diarrhea as a result of gastrointestinal distress, and tachycardia stemming from the sympathomimetic effects of the substance.

Weight gain is not a sign of metaldehyde toxicity. In fact, toxicity often leads to weight loss or failure to thrive due to the animal's compromised health and gastrointestinal issues. The other listed signs are directly tied to the pharmacological actions of metaldehyde, which induces increased metabolism and stress on bodily systems, while weight gain would contradict the effects typically observed in cases of poisoning. Thus, recognizing that weight gain is not associated with this type of toxicity supports its identification as the correct answer.