## **Core Concept**
The question tests the understanding of referred pain in the context of head and neck cancer, specifically carcinoma of the base of the tongue. Referred pain occurs when pain is perceived at a location other than the site of the painful stimulus. This phenomenon is due to the convergence of afferent nerve fibers from different regions onto the same spinal cord segment or cranial nerve nucleus.

## **Why the Correct Answer is Right**
The correct answer, **Glossopharyngeal nerve (CN IX)**, is right because this nerve provides sensory innervation to the base of the tongue. The glossopharyngeal nerve also has a branch, the tympanic nerve (Jacobson's nerve), which supplies the middle ear. Through the tympanic plexus, pain from the base of the tongue can be referred to the ear. This is an example of referred pain due to the shared nerve supply.

## **Why Each Wrong Option is Incorrect**
- **Option A: Vagus nerve (CN X)**: While the vagus nerve does provide some sensory innervation to the larynx and other areas of the head and neck, it is not primarily responsible for the sensory innervation of the base of the tongue or the referred pain to the ear in this context.
- **Option B: Trigeminal nerve (CN V)**: The trigeminal nerve provides sensory innervation to the face but is not directly involved in the innervation of the tongue or the referred pain pathway from the base of the tongue to the ear.
- **Option D: Hypoglossal nerve (CN XII)**: The hypoglossal nerve is primarily a motor nerve supplying the muscles of the tongue and does not have a role in sensory innervation or referred pain.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the **glossopharyngeal nerve** is involved in the sensory innervation of the base of the tongue and the referred pain to the ear in cases of carcinoma of the base of the tongue. This is a classic example of referred pain in clinical practice.

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