**Core Concept**
The fastest route of absorption of local anaesthetics is through the venous system, specifically the **intravascular route**. This is because local anaesthetics are highly lipophilic and can easily cross the lipid bilayer of cell membranes, facilitating rapid diffusion into the bloodstream.

**Why the Correct Answer is Right**
Local anaesthetics can be absorbed through various routes, including intravascular, extravascular, and mucosal. However, the intravascular route is the fastest due to the high concentration of local anaesthetic in the bloodstream. This is because local anaesthetics are highly protein-bound, which allows them to be rapidly taken up by the bloodstream. Additionally, the presence of blood flow in the venous system facilitates the rapid diffusion of local anaesthetics into the bloodstream.

**Why Each Wrong Option is Incorrect**
**Option A:** Intramuscular injection is not the fastest route of absorption, as local anaesthetics are initially taken up by the muscle cells before being released into the bloodstream.
**Option B:** Subcutaneous injection is slower than intravascular absorption, as local anaesthetics need to diffuse through the interstitial tissue before entering the bloodstream.
**Option C:** Mucosal absorption is slower and less efficient than intravascular absorption, as local anaesthetics need to diffuse through the mucous membrane and be taken up by the bloodstream.

**Clinical Pearl / High-Yield Fact**
Local anaesthetics can cause systemic toxicity if absorbed too quickly, leading to central nervous system and cardiovascular effects. Therefore, it is essential to use the slowest effective concentration and administration technique to minimize the risk of systemic toxicity.

**Correct Answer: C. Intravascular.