Which of the following is a sign of cranial nerve V dysfunction?

• A. Ptosis
• B. Hearing loss
• C. Weakness of the jaw
• D. Loss of smell

Answer: (C)

Understanding that cranial nerve V mainly controls the muscles of mastication and facial sensation explains why a weak jaw points to trigeminal nerve dysfunction. The motor fibers supply the masseter, temporalis, and pterygoid muscles; when CN V function is impaired, these jaw-closing muscles weaken, making biting hard and often causing the jaw to deviate toward the weaker side when opening. While CN V issues can also bring facial sensory loss or an altered jaw-jerk reflex, the standout sign is weakness of the jaw. Ptosis indicates oculomotor nerve problems, hearing loss involves the vestibulocochlear nerve, and loss of smell involves the olfactory nerve, none of which reflect CN V.