A patient has decreased sensation in the oral cavity with intact taste. Which cranial nerve is most likely impaired?

• A. Cranial nerve V (Trigeminal)
• B. Cranial nerve VII (Facial)
• C. Cranial nerve IX (Glossopharyngeal)
• D. Cranial nerve XII (Hypoglossal)

Answer: (A)

General somatic sensation from the oral cavity (touch, pain, temperature) is carried by the trigeminal nerve, while taste comes from other nerves in specific tongue regions (facial for the anterior two-thirds via chorda tympani, glossopharyngeal for the posterior third, and vagus for other areas). When sensation in the oral cavity is reduced but taste remains intact, the pattern fits an issue with the somatic sensory pathway rather than with taste pathways. The trigeminal nerve, especially its lingual branch that conveys general sensation from the anterior two-thirds of the tongue and floor of the mouth, would explain decreased oral sensation without affecting taste.

The other nerves are more tied to taste or motor function: facial carries taste from the anterior tongue, glossopharyngeal handles taste and sensation from the posterior tongue, and hypoglossal controls tongue movement but not sensory input. Therefore the scenario points to an impairment of the trigeminal nerve.