**Core Concept**
Infiltration anaesthesia is a local anaesthetic technique where a local anaesthetic agent is injected directly into the tissue to block nerve conduction, thereby relieving pain in a specific area. This method is commonly used for minor surgical procedures, such as skin biopsies or minor lacerations.

**Why the Correct Answer is Right**
When a local anaesthetic agent is injected directly into the tissue, it diffuses out of the injection site and blocks the nerve endings in the surrounding area. This blocks the transmission of pain signals from the nerve endings to the central nervous system, resulting in numbness or analgesia in the affected area. The diffusion of the local anaesthetic agent into the surrounding tissue is the key mechanism behind infiltration anaesthesia.

**Why Each Wrong Option is Incorrect**
**Option B:** Nerve block involves the injection of a local anaesthetic agent around a specific nerve or group of nerves to block pain signals, but it does not involve direct injection into the tissue.

**Option C:** Field block involves the injection of a local anaesthetic agent around a specific area or "field" to block pain signals, but it typically involves a larger volume of anaesthetic agent than infiltration anaesthesia and is often used for more extensive procedures.

**Option D:** Bier's block involves the intravenous injection of a local anaesthetic agent with a tourniquet applied to the limb above the injection site, which is not a direct injection into the tissue.

**Clinical Pearl / High-Yield Fact**
It's essential to note that infiltration anaesthesia is typically used for minor surgical procedures, and the volume of local anaesthetic agent used should be carefully calculated to avoid toxicity. A general rule of thumb is to use a maximum volume of 5-7mg/kg of lidocaine for infiltration anaesthesia.

**✓ Correct Answer: A. Infiltration anaesthesia**