All of the following are properties of local anaesthetics EXCEPT :
Local anesthetics work by blocking voltage-gated sodium channels, preventing action potentials. Their properties include lipid solubility, which affects their potency and duration. They also have a pKa that determines their ionization state—non-ionized form can cross cell membranes. The question is missing options, but common incorrect properties might involve things like being acidic, having high pKa, or being water-soluble. Wait, local anesthetics are typically weak bases with pKa around 7.5-9. So if an option says they're acidic, that's wrong. Also, they have a molecular structure with an aromatic ring, intermediate chain, and tertiary amine. If an option mentions something like "they are all esters," that's incorrect because some are amides. So the exceptions could be related to structure, pH, or solubility. Let me think of the usual properties to identify the exception. For example, if an option states that they have a pKa higher than 9, that's incorrect. Or if they don't have a tertiary amine, that's wrong. Another possible exception is if they are not metabolized by plasma esterases, which is true for amides. So if an option says all are metabolized by plasma esterases, that's incorrect. The clinical pearl here is that local anesthetics are weak bases with pKa around 7.5-9, and their ionization affects their ability to block sodium channels. The correct answer would be the option that contradicts these properties.
**Core Concept**
Local anesthetics block voltage-gated sodium channels, preventing action potential propagation. Key properties include lipid solubility, pKa, and molecular structure (e.g., aromatic ring, intermediate chain, tertiary amine). Their ionization state (non-ionized vs. ionized) affects membrane penetration and potency.
**Why the Correct Answer is Right**
Local anesthetics are **weak bases** with pKa values between 7.5 and 9. At physiological pH (7.4), they exist as a mixture of ionized (charged) and non-ionized (uncharged) forms. The non-ionized form diffuses across lipid membranes, while the ionized form blocks sodium channels intracellularly. This dual requirement for ionization and lipid solubility is critical for their mechanism of action.
**Why Each Wrong Option is Incorrect**
**Option A:** *If claiming local anesthetics are acidic* – Incorrect. They are weak bases, not acids.
**Option B:** *If stating they lack tertiary amine* – Incorrect. All local anesthetics (e.g., lidocaine, bupivacaine) have a tertiary amine in their structure.
**Option C:** *If asserting they are metabolized solely by esterases* – Incorrect. Amide-type local anesthetics (e.g., lidocaine) are metabolized by the liver, not plasma esterases.
**Clinical Pearl / High-Yield Fact**
Remember: **"Local anesthetics are weak bases with pKa ~7.5–9."** Their ionization at physiological pH creates a "