A 40-year-old woman received a left inferior alveolar nerve block for a restoration on tooth #18. Since then she cannot taste anything on the left side of her tongue. Referring to the diagram, in which regions would the loss of taste be expected?

Correct answer: B. Regions 2 and 3 (left anterior two-thirds)

• A. Adding region 4 incorrectly extends the deficit into CN IX territory.
• B. Correct. The IAN block anesthetizes the contents of the pterygomandibular space, which include the LINGUAL nerve. Travelling with the lingual nerve is the CHORDA TYMPANI (a branch of the facial nerve, CN VII), which carries taste from the IPSILATERAL anterior two-thirds of the tongue. Prolonged lingual nerve paresthesia therefore abolishes taste on the left anterior two-thirds only — regions 2 and 3 on this diagram.
• C. Regions 1 and 2 would represent bilateral loss; a unilateral block affects only the injected side.
• D. The posterior third is supplied by the glossopharyngeal nerve (CN IX), which is well outside the field of an IAN block.

Note: Taste map: anterior 2/3 → chorda tympani (CN VII); posterior 1/3 → CN IX; epiglottic region → CN X. General sensation to the anterior 2/3 is the lingual nerve (V3).