Sensory innervation of the ROOT (base) of the tongue, posterior to the epiglottic region, is supplied by which nerve?

Correct answer: C. The internal laryngeal branch of the vagus nerve (CN X)

• A. The lingual nerve (V3) carries general sensation from the ANTERIOR TWO-THIRDS of the tongue, and conveys taste from that region via the chorda tympani. It does not reach the root.
• B. CN IX supplies both general sensation AND taste for the POSTERIOR ONE-THIRD of the tongue, up to but not including the most posterior root/vallecular region. It is the standard answer for 'posterior third,' but 'root of tongue' specifically belongs to the vagus.
• C. CORRECT. The most posterior part of the tongue — the ROOT, in the region of the vallecula and lingual surface of the epiglottis — is supplied by the INTERNAL LARYNGEAL BRANCH of the superior laryngeal nerve, a branch of the VAGUS (CN X). This is the afferent limb of the gag and cough reflexes in that zone.
• D. CN XII is purely MOTOR to all intrinsic and extrinsic tongue muscles except palatoglossus. It carries no sensation at all.

Note: Full map worth memorising — Anterior 2/3: general sensation = lingual (V3), taste = chorda tympani (VII). Posterior 1/3: general AND taste = glossopharyngeal (IX). Root/vallecula: internal laryngeal branch of vagus (X). Motor: hypoglossal (XII), except palatoglossus (vagus, via pharyngeal plexus).