Which of the following factors is least likely to cause toxicity from a local anaesthetic injection?

Injecting in a supine position is least likely to cause toxicity from a local anaesthetic injection because this position does not inherently increase the risk of the anaesthetic agent entering the vascular system or leading to systemic toxicity.

When a patient is in a supine position, the absorption and distribution of the local anaesthetic may be more uniform and does not typically pose a direct threat of rapid systemic uptake compared to other factors. In contrast, injecting in a vascular area can introduce the anaesthetic directly into the bloodstream, increasing the risk of toxicity. Injecting without a vasoconstrictor can lead to a more rapid systemic absorption of the anaesthetic, also raising toxicity concerns. Intravenous injections, by their nature, directly inject the drug into the bloodstream, which can lead to immediate and potentially severe systemic effects.

Thus, the supine position is a neutral factor that does not exacerbate the risks associated with local anaesthetic injections in the same way that the other options do.