**Core Concept:** Topical anaesthesia refers to the local numbness induced by the application of a local anaesthetic agent to a specific body area, in this case, lignocaine. It is used to block sensory nerves and interrupt pain transmission. Lignocaine is a local anaesthetic agent that belongs to the class of amide local anaesthetics.

**Why the Correct Answer is Right:** The concentration of lignocaine used for topical anaesthesia depends on the desired depth and duration of anaesthesia, as well as the patient's age, weight, and overall health condition. Generally, for adults, a concentration of 1% to 2% is used. This concentration provides an effective balance between the desired degree of anaesthesia and minimal systemic absorption.

**Why Each Wrong Option is Incorrect:**

A. A concentration of 0.5% might not provide sufficient anaesthesia due to its lower potency.
B. A concentration of 3% is too high and might cause significant systemic absorption leading to systemic toxicity.
C. A concentration of 4% is too high, similar to option B, and may cause toxicity.
D. A concentration of 5% is too high and is likely to cause significant systemic absorption and toxicity.

**Why the Correct Answer is Right:** 1% to 2% lignocaine concentration provides a balance between desired anaesthesia and minimizing systemic absorption, ensuring patient safety and efficiency.

**Clinical Pearl:** When choosing the concentration of local anaesthetics for topical administration, consider factors like patient characteristics, clinical requirements, and potential risks. Maintaining a balance between effective anaesthesia and minimal systemic absorption is essential to ensure patient safety and optimal treatment outcomes.