**Core Concept**
Local anaesthetics block nerve conduction by inhibiting voltage-gated sodium channels in the axon, preventing action potential generation. This blockade requires the drug to enter the neuronal cell membrane to bind to these channels, particularly in the node of Ranvier.

**Why the Correct Answer is Right**
Local anaesthetics must diffuse across the plasma membrane of the neuron to reach their site of action—voltage-gated sodium channels in the axon. Only after crossing the membrane can they bind to the intracellular domain of these channels and stabilize them in a closed state, preventing depolarization. This membrane permeability is a fundamental requirement for effective blockade.

**Why Each Wrong Option is Incorrect**
Option A: Resting nerves are not more susceptible to blockade than stimulated nerves; in fact, the depolarized state during stimulation makes sodium channels less available for binding.
Option C: Smaller myelinated fibers are blocked before large ones due to their higher density of sodium channels and shorter conduction velocity, not the reverse.
Option D: Local anaesthetics do not alter the resting membrane potential; they prevent action potential generation without changing the baseline voltage.

**Clinical Pearl / High-Yield Fact**
The onset of action of local anaesthetics depends on their ability to cross the lipid membrane—this is why lipophilicity and pKa are critical in determining potency and duration.

✓ Correct Answer: B. Need to cross the cell membrane to produce the block