**Question:** All of the following statements about lignocaine are true except

A. Lignocaine is a local anaesthetic drug.
B. Lignocaine does not cross the blood-brain barrier.
C. Lignocaine causes a decrease in heart rate and blood pressure.
D. Lignocaine is a calcium channel blocker.

**Correct Answer: D. Lignocaine is a calcium channel blocker.**

**Core Concept:**
Lignocaine, also known as lidocaine, is a commonly used local anaesthetic drug within the class of amide local anaesthetics. These drugs work by blocking sodium channels in the neuronal membrane, preventing the initiation and conduction of nerve impulses, thus causing local anaesthesia. Calcium channel blockers, on the other hand, are medications that work by inhibiting the movement of calcium ions across cell membranes, mainly targeting vascular smooth muscle cells and cardiac myocytes. Lignocaine is not a calcium channel blocker.

**Why the Correct Answer is Right:**
Lignocaine is a local anaesthetic, not a calcium channel blocker. Its primary action is on peripheral nerves and not on vascular smooth muscle or cardiac myocytes. It specifically blocks voltage-gated sodium channels in the neuronal membrane, preventing the depolarization and propagation of action potentials. This property is crucial for its function as a local anaesthetic.

**Why Each Wrong Option is Incorrect:**
A. While lignocaine is indeed a local anaesthetic drug, this statement is true and does not contradict the correct answer.
B. Lignocaine does cross the blood-brain barrier and can reach the central nervous system. This is incorrect, making this statement false.
C. Lignocaine can cause bradycardia (slow heart rate) and hypotension (low blood pressure) due to its cardiac, respiratory, and central nervous system effects. However, this statement is not directly related to the question about calcium channel blocking properties and is thus not relevant.
D. As explained above, lignocaine is a calcium channel blocker. This statement is false and contradicts the correct answer.

**Clinical Pearls:**
While lignocaine has various clinical applications, it is essential to differentiate its primary function as a local anaesthetic from its incorrect function as a calcium channel blocker. This distinction is crucial for understanding its mechanism of action and potential side effects.