## **Core Concept**
The posterior third of the tongue receives its sensory innervation from a specific cranial nerve. The tongue's sensory innervation varies along its length, with different sections supplied by different nerves. The posterior third is primarily concerned with general sensation, including taste.

## **Why the Correct Answer is Right**
The correct answer, glossopharyngeal nerve (cranial nerve IX), is responsible for the sensory innervation of the posterior third of the tongue. This includes both general sensation (touch, pressure, temperature) and special sensation (taste). The glossopharyngeal nerve achieves this through its lingual branch, which provides the sensory fibers.

## **Why Each Wrong Option is Incorrect**
- **Option A:** The **chorda tympani** (a branch of the facial nerve, cranial nerve VII) provides taste sensation to the anterior two-thirds of the tongue, not the posterior third.
- **Option B:** The **trigeminal nerve** (cranial nerve V) provides general sensation to the face, including the anterior two-thirds of the tongue (via the lingual nerve), but not the posterior third.
- **Option C:** The **vagus nerve** (cranial nerve X) provides innervation to the pharynx and larynx and is involved in the sensation of the epiglottis and the back of the throat but not specifically the posterior third of the tongue.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is the innervation of the tongue: the **anterior two-thirds** are supplied by the **chorda tympani** (taste) and the **lingual nerve** (general sensation), while the **posterior third** is supplied by the **glossopharyngeal nerve** (both taste and general sensation). This distinction is crucial for neurological and ENT examinations.

## **Correct Answer:** D. Glossopharyngeal nerve.