**Core Concept:** Traumatic dental injuries, specifically avulsion (tooth dislodgement) or luxation (tooth displacement), can lead to discolouration and root canal obliteration. Trauma can result in various dental injuries, some of which include discolouration and root canal obliteration.

**Why the Correct Answer is Right:** This case describes a patient with yellowish discoloration of a maxillary canine tooth due to trauma. The tooth is not tender on palpation or percussion test, suggesting that the discolouration is not due to infection or other dental pathology but rather a consequence of trauma. The radiograph reveals obliteration of the root canal, further supporting the diagnosis of a traumatic dental injury.

**Why Each Wrong Option is Incorrect:**

A. Pulp necrosis (Option A) - Pulp necrosis occurs due to infection or inflammation, not trauma, and would be indicated by tenderness on palpation and percussion test.
B. Endodontic infection (Option B) - Infection would result in tooth tenderness and pulsation, not discolouration and root canal obliteration.
C. Periodontal disease (Option C) - Periodontal disease would cause gingival inflammation and periodontal pocket formation, not tooth discolouration and root canal obliteration.
D. Tooth decay (Option D) - Tooth decay typically causes cavities and sensitivity, not discolouration and root canal obliteration.

**Clinical Pearl:** Traumatic dental injuries should be suspected when examining patients with dental discoloration and root canal obliteration, especially if there is a history of trauma. Early diagnosis and treatment can prevent further complications like pulp necrosis or periodontal disease and preserve the tooth structure.