Which of the following is a common vasoconstrictor used with local anesthetics?

Epinephrine is a common vasoconstrictor used in conjunction with local anesthetics to prolong their effects and minimize systemic absorption. By constricting blood vessels in the area where the local anesthetic is administered, epinephrine helps to maintain a higher concentration of the anesthetic at the target site, which enhances the duration of anesthesia and can also reduce bleeding during surgical procedures.

Local anesthetics often have a shorter duration of action, and using a vasoconstrictor like epinephrine not only extends their effectiveness but also mitigates potential side effects associated with rapid absorption into the systemic circulation. The other options listed do not serve as vasoconstrictors in this context. Fentanyl is an opioid analgesic, nitrous oxide is an anesthetic gas, and aspirin is an antiplatelet medication and does not have vasoconstrictive properties. Thus, epinephrine stands out as the agent used specifically for enhancing the performance of local anesthetics.