**Local Anaesthetics**

### Core Concept

Local anaesthetics are a class of medications that work by blocking the conduction of nerve impulses, thereby preventing the sensation of pain. They achieve this by inhibiting the voltage-gated sodium channels in neuronal membranes, which are essential for the initiation and propagation of action potentials.

### Why the Correct Answer is Right

The mechanism of action of local anaesthetics involves the binding of the drug to the sodium channel, which causes a conformational change that prevents the channel from opening. This results in a decrease in the influx of sodium ions into the neuron, leading to a decrease in the excitability of the neuron and ultimately, the prevention of pain sensation. The potency of local anaesthetics is influenced by their lipid solubility, with more lipid-soluble drugs being more potent.

### Why Each Wrong Option is Incorrect

**Option A:** This option is incorrect because local anaesthetics do indeed have a rapid onset of action, typically within 1-2 minutes of administration.

**Option B:** This option is incorrect because local anaesthetics do indeed have a duration of action that can range from a few minutes to several hours, depending on the specific drug and concentration used.

**Option C:** This option is incorrect because local anaesthetics are metabolized in the liver by enzymes such as cytochrome P450, but this is not the primary reason they are not true of local anaesthetics.

### Clinical Pearl / High-Yield Fact

It's worth noting that the addition of a vasoconstrictor such as epinephrine to local anaesthetics can prolong their duration of action by reducing blood flow to the area and slowing the absorption of the drug.

### Correct Answer: C. Local anaesthetics are primarily metabolized in the liver by enzymes.