Which one of the following local anaesthetics is an ester of benzoic acid?

RRB Staff Nurse Mumbai-2015

Pharmacology

Medium

Appeared in: RRB Staff Nurse Mumbai-2015

Explanation

Cocaine is a naturally occurring alkaloid and is chemically classified as an ester of benzoic acid.
Unlike most other local anesthetics, cocaine also possesses significant vasoconstrictive properties by inhibiting norepinephrine reuptake.
The ester linkage in cocaine is what categorizes it within the ester group of local anesthetics, distinguishing it from the amide group.

Option A: Lidocaine is a member of the amide class of local anesthetics, not the ester class. Its chemical structure contains an amide linkage.
Option B: Although Procaine is an ester-type local anesthetic, it is an ester of para-aminobenzoic acid (PABA), not benzoic acid. This is a key chemical distinction.
Option C: Ropivacaine, similar to lidocaine, is an amide-type local anesthetic. It contains an amide bond in its structure.

Visual 1: Diagram: Chemical structures of the four options (Lidocaine, Procaine, Ropivacaine, Cocaine), highlighting the amide linkage in lidocaine and ropivacaine, the PABA ester linkage in procaine, and the benzoic acid ester linkage in cocaine.
Visual 2: Flowchart: Illustrating the metabolic pathways for ester-type (plasma cholinesterase) versus amide-type (liver metabolism) local anesthetics.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Classification of Local Anesthetics as background academic context rather than a clinical decision trigger.
Understanding the difference between ester and amide local anesthetics is critical for patient safety, particularly regarding allergies. A documented allergy to an ester-type anesthetic (often due to the PABA metabolite) necessitates the use of an amide-type anesthetic.
The vasoconstrictive property of cocaine is unique among local anesthetics and is the reason for its historical and limited current use in topical anesthesia for procedures of the upper respiratory tract, as it reduces bleeding.
What if a patient has a history of liver failure? The metabolism of amide local anesthetics (like lidocaine and ropivacaine) would be impaired, increasing the risk of systemic toxicity. In such cases, an ester-type anesthetic might be considered, or the dose of the amide would need to be significantly reduced.

How to Approach the Question

First, identify the key terms in the question: 'local anaesthetics' and 'ester of benzoic acid'. This indicates the question is about the chemical classification of drugs.
Recall the two main classes of local anesthetics: esters and amides.
Mentally or on paper, classify each option. A useful mnemonic is that amides usually have two 'i's in their name (lidocaine), while esters have one (procaine).
For the ester options, differentiate further based on the acid they are derived from. Procaine is the classic example of a PABA ester.
Recognize that cocaine is the specific local anesthetic that is an ester of benzoic acid.

Concept Tested & Keywords

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