Damage to which of the following would NOT produce numbness of the upper lip?

Correct answer: D. A nerve emerging from the stylomastoid foramen

• A. Heavy infiltration in the anterior maxillary vestibule deposits solution around the terminal branches of the infraorbital nerve (ASA/MSA), producing upper lip numbness — this is the expected, intended effect of an infraorbital-region injection.
• B. The INFRAORBITAL nerve is the terminal continuation of V2 and supplies the upper lip, lateral nose, lower eyelid and labial mucosa. Damage here absolutely causes upper lip numbness.
• C. Surgery in the parotid/infraorbital region can injure the branches of the infraorbital nerve or the facial nerve. Where the infraorbital nerve or its branches are involved, sensory loss of the upper lip follows.
• D. CORRECT — this is the EXCEPTION. The nerve exiting the STYLOMASTOID FORAMEN is the FACIAL NERVE (CN VII), which is purely MOTOR to the muscles of facial expression in this context. Injury produces a droopy, immobile lip that the patient may describe as feeling odd, but it causes no true SENSORY loss. Upper lip SENSATION is carried by V2 via the infraorbital nerve.

Note: The clean distinction: CN VII = MOVEMENT of the lip (stylomastoid foramen). CN V2 = SENSATION of the lip (infraorbital foramen). A patient who cannot move the lip but feels it has a facial nerve problem; a patient who feels nothing but moves normally has a trigeminal problem.