**Core Concept**
Local anaesthetics are classified into two main groups: esters and amides, based on their chemical structure. Ester local anaesthetics are metabolized by plasma cholinesterases, whereas amide local anaesthetics are metabolized by the liver. This classification has implications for their duration of action and potential side effects.

**Why the Correct Answer is Right**
Procaine is an ester local anaesthetic that is primarily metabolized by plasma cholinesterases. Its ester linkage is susceptible to hydrolysis by this enzyme, resulting in the formation of para-aminobenzoic acid (PABA) and diethylaminoethanol (DEAE). This rapid metabolism contributes to procaine's short duration of action. In contrast, amide local anaesthetics like lignocaine and mepivacaine are metabolized by the liver and have longer durations of action.

**Why Each Wrong Option is Incorrect**
**Option B:** Bupivacaine is an amide local anaesthetic, not an ester. It is known for its long duration of action and potential for central nervous system toxicity.
**Option C:** Lignocaine is also an amide local anaesthetic, commonly used for infiltration, nerve blocks, and epidural anaesthesia. Its amide linkage is resistant to hydrolysis by plasma cholinesterases.
**Option D:** Mepivacaine is another amide local anaesthetic, similar to lignocaine in terms of its pharmacological properties and duration of action.

**Clinical Pearl / High-Yield Fact**
When administering local anaesthetics, it is essential to consider their metabolism and potential interactions with other medications. Ester local anaesthetics like procaine may interact with certain antibiotics, such as penicillins, which can be metabolized by the same enzymes.

**✓ Correct Answer: A. Procaine**