**Core Concept**
The patient's symptoms suggest a condition affecting the pharynx, causing pain upon swallowing, radiating to the ear and posterior part of the tongue. This presentation is characteristic of a specific type of pharyngitis, which involves the involvement of the sensory nerves of the pharynx.

**Why the Correct Answer is Right**
The correct diagnosis is **Option C:** Pharyngo-tonsillar abscess. This condition is caused by the extension of a tonsillar abscess into the pharynx, leading to severe pain upon swallowing, radiating to the ear and posterior part of the tongue. The pain is exacerbated by the involvement of the glossopharyngeal nerve (cranial nerve IX), which provides sensory innervation to the posterior part of the tongue and the pharynx.

The correct diagnosis is also supported by the patient's age, as pharyngo-tonsillar abscess is more common in adults. The condition often presents with a history of sore throat, fever, and difficulty swallowing.

**Why Each Wrong Option is Incorrect**
* **Option A:** Epiglottitis is a life-threatening condition that typically presents with severe sore throat, difficulty swallowing, and high fever. However, the patient's symptoms do not include difficulty breathing or stridor, which are characteristic of epiglottitis.
* **Option B:** Peritonsillar abscess is a condition characterized by an abscess formation on the tonsil itself, leading to severe pain, fever, and difficulty swallowing. However, the patient's symptoms do not radiate to the ear and posterior part of the tongue, which is more characteristic of pharyngo-tonsillar abscess.

**Clinical Pearl / High-Yield Fact**
The patient's symptoms should prompt the physician to assess the patient's airway, as pharyngo-tonsillar abscess can cause airway obstruction. A thorough examination, including a flexible laryngoscopy, may be necessary to evaluate the patient's airway and determine the need for urgent intervention.

**Correct Answer:** C. Pharyngo-tonsillar abscess.