Which is an example of an untoward reaction associated with regional anesthesia?

An intravascular injection of the anesthetic agent is a significant untoward reaction associated with regional anesthesia. This can happen when the anesthetic accidentally enters the bloodstream, which can lead to systemic toxicity. Symptoms may include cardiovascular changes such as hypotension, bradycardia, or even central nervous system effects such as seizures or unconsciousness. This reaction illustrates the importance of careful injection technique and monitoring to ensure patient safety during regional anesthesia procedures.

The other options, while they may occur in the context of anesthesia, are not specifically tied to regional anesthesia as untoward reactions in the same direct manner. Nausea and vomiting can occur across various types of anesthesia and are often related to postoperative factors, rather than to the type of anesthesia used. Excessive sedation is more common with general anesthesia than regional techniques, where the intent is to maintain consciousness in the patient. Allergic reactions can occur with any medication, including anesthetics, but again, they are not particularly characteristic of regional anesthesia alone.