**Question:** Which of the following local anaesthetic should not be alkalinized with soda bicarbonate?

**Core Concept:** Local anaesthetics are a class of medications used to block nerve signals and provide temporary analgesia. Soda bicarbonate, also known as sodium bicarbonate, is an alkaline solution used to neutralize acidic substances and increase pH. In the context of local anaesthetics, alkalinization can help improve their efficacy and reduce potential side effects like toxicity.

**Why the Correct Answer is Right:** Alkalinization can be beneficial for certain local anaesthetics, as it can enhance their penetration into nerve tissue and prolong their duration of action. However, not all local anaesthetics are suitable for alkalinization. In this case, we need to identify which local anaesthetic should not be alkalinized with soda bicarbonate.

**Why Each Wrong Option is Incorrect:**

A. **Lidocaine:** This is a commonly used local anaesthetic that can be safely alkalinized with soda bicarbonate, as it has been demonstrated to increase its duration of action and reduce the risk of toxicity.

B. **Bupivacaine:** Similar to lidocaine, bupivacaine can be alkalinized with soda bicarbonate for enhanced efficacy and reduced toxicity.

C. **Mepivacaine:** This local anaesthetic is also suitable for alkalinization, as it can benefit from increased pH and reduced toxicity when combined with soda bicarbonate.

D. **Ropivacaine:** Ropivacaine can be alkalinized with soda bicarbonate, although it is less commonly used compared to the other options due to its safety profile.

**Correct Answer:** Given that all options A, B, C, and D are suitable for alkalinization with soda bicarbonate to enhance their efficacy and decrease toxicity risks, the correct answer is:

E. **Benzocaine:** Benzocaine is not intended for alkalinization with soda bicarbonate due to its inherent toxicity and safety concerns. Alkalinization can worsen its pharmacokinetics and pharmacodynamics, potentially leading to adverse effects and decreased clinical utility.

**Why Benzocaine is Incorrect:** Benzocaine, an ester local anaesthetic, has several drawbacks compared to amide local anaesthetics (Lidocaine, Bupivacaine, Mepivacaine, and Ropivacaine). Benzocaine is highly toxic, with a narrow therapeutic index, and can cause serious adverse effects such as methemoglobinemia and methemoglobinemia. Alkalinization with soda bicarbonate can worsen these toxicities, making Benzocaine unsuitable for alkalinization compared to the other options.

**Clinical Pearl:** It is essential for medical professionals to understand the pharmacological properties and potential risks of different local anaesthetics to ensure patient safety and appropriate usage. In this context, knowledge of the safety profile and limitations of