**Core Concept**
Acute apical periodontitis is an inflammatory condition of the periapical tissues surrounding a tooth's apex, often resulting from pulpitis or pulp necrosis. Diagnosing this condition requires a method that can detect the associated inflammation and pain.

**Why the Correct Answer is Right**
Percussion is a clinical test where a dentist gently taps the tooth with a mallet or an instrument. In the case of acute apical periodontitis, this can cause pain due to the inflammation of the periapical tissues. The pain is typically localized to the area of the affected tooth and can be a reliable indicator of the condition. The periapical tissues are highly innervated with nociceptors, which are sensitive to pressure and vibrations, making percussion a sensitive test for detecting inflammation.

**Why Each Wrong Option is Incorrect**
**Option B:** Thermal tests, such as cold or heat tests, are more suited for detecting pulp vitality rather than periapical pathology. They are less effective in diagnosing acute apical periodontitis.
**Option C:** Electric pulp tests can also detect pulp vitality, but they are not as effective in diagnosing periapical pathology. They may not produce a response in the case of acute apical periodontitis.
**Option D:** Anaesthetic test, also known as a pulp vitality test, is used to determine the vitality of the pulp. It is not effective in diagnosing acute apical periodontitis, which is a periapical condition.

**Clinical Pearl / High-Yield Fact**
When performing percussion tests, dentists should use a gentle and controlled motion to avoid causing unnecessary pain or discomfort to the patient. A positive response to percussion, such as pain or tenderness, can be an indicator of acute apical periodontitis.

**✓ Correct Answer: A. Percussion. Percussion is a clinical test used to diagnose acute apical periodontitis by detecting pain or tenderness in the periapical tissues.**