Which of the following is associated with a significant risk of toxicity when ingested?

Fluoride is associated with a significant risk of toxicity when ingested due to its potential to cause acute and chronic health issues. Ingestion of fluoride in high amounts can lead to symptoms such as nausea, vomiting, abdominal pain, and in severe cases, can lead to systemic toxicity involving cardiovascular and central nervous system effects. The mechanism of toxicity is primarily through its ability to interfere with calcium metabolism and the integrity of bones and teeth, potentially leading to conditions such as fluorosis or even acute fluoride poisoning in significant cases.

The other substances listed do not carry the same level of acute toxicity upon ingestion. Calcium acetate is used primarily as a phosphate binder and has a relatively low toxicity profile when ingested in recommended amounts. Sodium bicarbonate can cause metabolic alkalosis in excessive doses, but is often used as an antacid and in emergency settings for metabolic acidosis. Potassium bromide has been historically used as an anticonvulsant but its safety profile is much better understood, and serious toxicity is less common compared to fluoride. Therefore, fluoride stands out as the substance with a higher risk of acute toxicity upon ingestion.