What could cause one side of a patient's face to "droop" after an injection?

A droop on one side of a patient's face after an injection can occur if the local anesthetic is inadvertently deposited into the parotid gland. This is particularly relevant when administering an inferior alveolar nerve block. If the anesthetic reaches the parotid gland, it can affect the nearby facial nerve (cranial nerve VII), which controls the muscles of facial expression. The temporary paralysis of this nerve leads to a characteristic drooping appearance on the affected side of the face.

Understanding this process is crucial for dental professionals to avoid complications during injections. While other scenarios presented in the options may lead to discomfort or complications, they do not typically result in facial drooping due to direct involvement with the facial nerve. For example, muscle spasms or syncope would not directly affect facial nerve function in that manner, and a needle nicking a blood vessel usually causes localized swelling or bruising rather than facial asymmetry.