Bell's palsy most commonly involves which cranial nerve?

• A. Facial nerve (CN VII)
• B. Optic nerve (CN II)
• C. Olfactory nerve (CN I)
• D. Auditory nerve (CN VIII)

Answer: (A)

Bell's palsy is defined by inflammation and acute weakness of the muscles of facial expression due to involvement of the facial nerve as it travels through the facial canal. This nerve, CN VII, carries motor fibers that innervate the muscles of facial expression, so its dysfunction produces the characteristic one-sided facial droop, inability to raise the eyebrow, close the eye, or smile on that side. It may also affect some sensory and parasympathetic branches, leading to altered taste in the anterior two-thirds of the tongue and changes in lacrimation or sensitivity to sounds, but the hallmark is the motor weakness of the face.

The other nerves listed do not control facial muscles: the optic nerve handles vision, the olfactory nerve handles smell, and the auditory (vestibulocochlear) nerve handles hearing and balance. Their dysfunction would present with vision loss, loss of smell, or hearing/balance problems, not the facial paralysis seen with Bell's palsy.