A patient with a dropped jaw and otherwise normal neurological exam likely has which condition?

The condition that best explains a dropped jaw in a patient with an otherwise normal neurological exam is trigeminal neuritis. The trigeminal nerve (cranial nerve V) is responsible for the motor function of the muscles of mastication, which includes the masseter, temporalis, and pterygoid muscles. When there is inflammation or dysfunction of this nerve, it can result in weakness of these muscles, causing the jaw to drop since the muscles are unable to provide the necessary tone to hold the jaw up.

In the context provided, a dropped jaw indicates that the muscles responsible for closing the jaw are not functioning properly. The normal neurological exam suggests that other cranial nerves and neurological functions are intact, supporting the diagnosis of isolated trigeminal nerve involvement rather than a more systemic or multifocal neurologic issue.

Facial neuritis, cranial nerve VII paralysis, and mandibular myopathy do not specifically explain the dropped jaw in the same way, as they involve different aspects of muscle function or involve other cranial nerves that may not result in the same presentation.