Local Anesthetics — MCQs
Which of the following local anesthetics is the most common cause of methemoglobinemia?
What is the preferred concentration range of lidocaine for topical anesthesia?
Which of the following is not an amide local anesthetic?
Interscalene approach to brachial plexus block does not provide optimal surgical anaesthesia in the area of distribution of which of the following nerve?
Which local anesthetic is considered the most cardiotoxic?
Which of the following is FALSE about ropivacaine?
Which of the following nerve fibre types is least susceptible to LA blockade?
The application of the following product can lead to development of: (Recent NEET Pattern 2016-17)
Carbapenem which has a tendency to cause maximum seizures?
What is the commonly used concentration of tetracaine for topical anesthesia in minor ophthalmic procedures?
Local Anesthetics Indian Medical PG Practice Questions and MCQs
Question 1: Which of the following local anesthetics is the most common cause of methemoglobinemia?
- A. Lignocaine
- B. Benzocaine (Correct Answer)
- C. Chloroprocaine
- D. EMLA Cream (Lignocaine + Prilocaine)
- E. Prilocaine
- F. Dibucaine
Explanation: ***Benzocaine***- **Benzocaine** is an ester-type local anesthetic that is the **most common cause of methemoglobinemia** among local anesthetics, especially when used in high doses or on mucous membranes due to its rapid absorption.- Its metabolic byproducts, particularly **aniline derivatives**, are potent oxidizers of hemoglobin, converting the ferrous iron (Fe2+) to ferric iron (Fe3+), thus forming methemoglobin which cannot bind oxygen.- **FDA warnings** have been issued regarding benzocaine-induced methemoglobinemia, particularly with topical spray preparations.*Lignocaine*- **Lignocaine** (lidocaine) is an amide-type local anesthetic and is **rarely associated** with methemoglobinemia.- While it can theoretically cause it in very high doses, it is significantly **less potent** in this regard compared to benzocaine.*Chloroprocaine*- **Chloroprocaine** is an ester-type local anesthetic with a very **short duration of action** due to rapid hydrolysis by plasma cholinesterases.- This rapid metabolism typically limits systemic exposure and makes it an **uncommon cause** of methemoglobinemia despite being an ester.*Prilocaine*- **Prilocaine** is an amide-type local anesthetic that can also cause methemoglobinemia, particularly at **higher doses (>600mg)** [1, 2].- It works through its metabolite, **o-toluidine**, which is an oxidizing agent [1].- However, **benzocaine** is more consistently linked to this adverse effect in clinical practice and has more documented case reports.
Question 2: What is the preferred concentration range of lidocaine for topical anesthesia?
- A. 2-4% (Correct Answer)
- B. 7-12%
- C. 12-15%
- D. <2%
Explanation: ***Correct Option: 2-4%*** - **Lidocaine** is an **amide-type local anesthetic** commonly used for topical anesthesia to numb localized areas before minor procedures. - The **preferred concentration range for topical application is 2-4%**, which provides effective anesthesia with an acceptable safety profile. - **2% lidocaine gel/cream** is commonly used for skin and genital mucosa. - **4% lidocaine** is standard for oral and respiratory mucous membranes. - **5% lidocaine patches** are used for post-herpetic neuralgia. - This concentration range balances **clinical efficacy** with **minimal systemic toxicity risk**. *Incorrect Option: <2%* - Concentrations below **2%** are generally **suboptimal** for achieving significant topical anesthesia. - These lower concentrations result in **insufficient pain relief** or require longer application times. - While 0.5-1% solutions exist, they are primarily used for infiltration anesthesia, not topical application. *Incorrect Option: 7-12%* - Concentrations in the **7-12%** range are **too high** for routine topical use and increase the risk of **systemic toxicity**. - These concentrations are not standard in clinical practice for general topical anesthesia. - Higher concentrations increase absorption without proportional improvement in efficacy. *Incorrect Option: 12-15%* - Concentrations in the **12-15%** range are **excessively high** and pose substantial **risk of systemic absorption and toxicity**. - Such high concentrations are **not recommended** for topical anesthesia in clinical practice. - Even 10% sprays (used for oropharyngeal anesthesia) require strict dose limitations to prevent toxicity.
Question 3: Which of the following is not an amide local anesthetic?
- A. Prilocaine
- B. Bupivacaine
- C. Lignocaine
- D. Cocaine (Correct Answer)
Explanation: ***Cocaine*** - **Cocaine** is an **ester-type** local anesthetic, not an amide. Ester local anesthetics are characterized by an **ester bond** in their chemical structure. - It works by blocking nerve impulse transmission and also by **inhibiting norepinephrine reuptake**, contributing to its vasoconstrictive and stimulatory effects. *Lignocaine* - **Lignocaine** (also known as **lidocaine**) is an **amide-type** local anesthetic, characterized by an **amide bond** in its chemical structure. - Amide local anesthetics are generally more stable and have a longer duration of action compared to ester types. *Prilocaine* - **Prilocaine** is an **amide-type** local anesthetic, commonly used in dentistry and for regional anesthesia. - Its amide structure contributes to its stability and intermediate duration of action. *Bupivacaine* - **Bupivacaine** is an **amide-type** local anesthetic known for its relatively long duration of action and potency. - It is often used for epidural anesthesia and nerve blocks due to its prolonged effect.
Question 4: Interscalene approach to brachial plexus block does not provide optimal surgical anaesthesia in the area of distribution of which of the following nerve?
- A. Median
- B. Musculocutaneous
- C. Radial
- D. Ulnar (Correct Answer)
Explanation: ***Ulnar*** - The **ulnar nerve** (C8-T1) emerges from the lower trunk of the brachial plexus. During an **interscalene block**, the local anesthetic is typically deposited at the level of the roots and trunks (C5-C7), which is superior to the origin of the lower trunk that gives rise to the ulnar nerve. - Due to the **cephalad spread** of the local anesthetic from an interscalene block, the **C8 and T1** nerve roots (and thus the ulnar nerve) are often not adequately blocked, leading to suboptimal anesthesia in its distribution. *Median* - The **median nerve** (C5-T1) originates from the lateral and medial cords, which are typically well-covered by the spread of local anesthetic in an interscalene block due to its formation from the middle and upper trunks. - Optimal anesthesia in the distribution of the median nerve is generally achieved with an interscalene block, as its nerve roots are within the targeted antegrade spread. *Musculocutaneous* - The **musculocutaneous nerve** (C5-C7) arises from the lateral cord, which is formed by the upper and middle trunks. These structures are reliably blocked during an interscalene approach. - Sensory and motor functions of the musculocutaneous nerve, such as **biceps contraction** and lateral forearm sensation, are usually well anesthetized. *Radial* - The **radial nerve** (C5-T1) is a branch of the posterior cord, which receives fibers from all three trunks. Its upper and middle trunk components are generally well-blocked by an interscalene approach. - While complete anesthesia of the entire brachial plexus can be variable, the radial nerve is more consistently affected by an interscalene block than the ulnar nerve due to its more extensive proximal root contributions which are within the typical spread.
Question 5: Which local anesthetic is considered the most cardiotoxic?
- A. Procaine
- B. Prilocaine
- C. Ropivacaine
- D. Bupivacaine (Correct Answer)
Explanation: ***Bupivacaine*** - **Bupivacaine** is an amide-type local anesthetic associated with significant **cardiotoxicity** due to its high lipid solubility and slow dissociation from cardiac sodium channels. - This can lead to severe **arrhythmias** and myocardial depression, making it particularly dangerous in systemic overdose. *Procaine* - **Procaine** is an ester-type local anesthetic with a relatively low potential for cardiotoxicity. - Its rapid metabolism by **plasma pseudocholinesterase** limits systemic exposure and reduces the risk of cardiac effects. *Prilocaine* - **Prilocaine** is an amide-type local anesthetic that is generally less cardiotoxic than bupivacaine. - Its primary concern is the potential to cause **methemoglobinemia** at higher doses, a side effect not directly related to cardiotoxicity. *Ropivacaine* - **Ropivacaine** is an amide-type local anesthetic developed as an alternative to bupivacaine with a reduced cardiotoxicity profile. - It exhibits a more favorable **therapeutic index** for cardiac effects due to its chemical structure and faster dissociation from cardiac sodium channels.
Question 6: Which of the following is FALSE about ropivacaine?
- A. More cardiotoxic than lignocaine
- B. Contains only R enantiomer (Correct Answer)
- C. Less cardiotoxic than bupivacaine
- D. Onset of action is faster than bupivacaine
Explanation: ***Contains only R enantiomer*** - Ropivacaine is a **pure S-enantiomer** (S-(-)-enantiomer) preparation, NOT the R-enantiomer. - This single enantiomer formulation contributes to its **reduced cardiotoxicity** and improved safety profile compared to racemic bupivacaine. - The statement "contains only R enantiomer" is **FALSE** - this is the correct answer. *Less cardiotoxic than bupivacaine* - Ropivacaine has **lower lipid solubility** and faster dissociation from cardiac sodium channels compared to bupivacaine. - This results in **significantly reduced cardiotoxicity**, making it safer for large-volume regional blocks. - This statement is **TRUE**. *More cardiotoxic than lignocaine* - Ropivacaine is indeed **more cardiotoxic than lidocaine** due to its greater potency and longer duration of action. - Amide local anesthetics with higher potency (bupivacaine > ropivacaine > lidocaine) carry greater cardiac risk. - This statement is **TRUE**. *Onset of action is faster than bupivacaine* - Ropivacaine and bupivacaine have **similar onset times** (10-15 minutes for epidural/nerve blocks). - Both have similar pKa values (ropivacaine 8.1, bupivacaine 8.1), resulting in comparable onset characteristics. - While ropivacaine may have marginally faster onset in some contexts, clinically they are considered equivalent. - This statement is generally **FALSE or equivocal**, making it potentially the second-best answer, but the R-enantiomer statement is definitively false.
Question 7: Which of the following nerve fibre types is least susceptible to LA blockade?
- A. B fibers
- B. A beta
- C. C fibers
- D. A alpha (Correct Answer)
Explanation: **A alpha** - **A alpha fibers** are the **largest** and most heavily myelinated nerve fibers, responsible for **motor function** and **proprioception**. - Due to their large diameter and thick myelination, they have the **highest conduction velocity** and are the **least susceptible to local anesthetic blockade**, requiring higher concentrations or longer exposure times. *B fibers* - **B fibers** are **preganglionic autonomic fibers** that are myelinated but of relatively small diameter. - They are generally **highly sensitive to local anesthetics**, often being blocked even before A-delta and C fibers, due to their specific physiologic properties like repetitive firing and length of node of Ranvier. *C fibers* - **C fibers** are **unmyelinated** and have the **smallest diameter**, transmitting **pain, temperature, and autonomic information**. - Despite being unmyelinated, their small diameter makes them **highly sensitive to local anesthetics**, as the drug can easily penetrate to block sodium channels. *A beta* - **A beta fibers** are large, myelinated fibers involved in transmitting **touch and pressure sensations**. - While myelinated, they are **smaller than A-alpha fibers** and thus more susceptible to local anesthetic blockade than A-alpha, but less so than C or B fibers.
Question 8: The application of the following product can lead to development of: (Recent NEET Pattern 2016-17)
- A. Seizures
- B. Methemoglobinemia (Correct Answer)
- C. Depression
- D. Hypertension
Explanation: ***Methemoglobinemia (Correct)*** - EMLA cream contains **lidocaine** and **prilocaine**, both of which are **amide-type local anesthetics** - Prilocaine, in particular, can cause methemoglobinemia due to its metabolite, **o-toluidine**, which oxidizes hemoglobin to methemoglobin - While methemoglobinemia is rare with proper use, it is a known and serious adverse effect, especially when applied to large areas, on broken skin, or in infants - This is the most specific adverse effect associated with prilocaine in EMLA cream *Seizures (Incorrect)* - Seizures are a known systemic toxicity of local anesthetics like lidocaine and prilocaine but are usually associated with **very high systemic concentrations**, often from accidental intravenous injection or excessive application over large areas - While possible with overdose, methemoglobinemia is a more specific and direct concern linked to prilocaine's metabolic pathway - CNS toxicity typically requires systemic absorption beyond what occurs with proper topical use *Depression (Incorrect)* - Depression is **not a direct or common adverse effect** of EMLA cream or its components - Systemic absorption of local anesthetics can cause central nervous system effects, but depression is generally not among them - CNS effects more commonly involve excitation (restlessness, tremors) or sedation depending on the dose *Hypertension (Incorrect)* - Local anesthetics generally cause **vasodilation** at therapeutic doses, potentially leading to **hypotension** rather than hypertension, especially with systemic absorption - Hypertension is not a characteristic side effect associated with the topical application of EMLA cream - The cardiovascular effects of local anesthetics, when present, typically involve decreased blood pressure and myocardial depression
Question 9: Carbapenem which has a tendency to cause maximum seizures?
- A. Imipenem (Correct Answer)
- B. Ertapenem
- C. Doripenem
- D. Meropenem
Explanation: ***Imipenem*** - **Imipenem** is associated with the highest risk of **seizures** among the carbapenems, particularly in patients with **renal impairment**, pre-existing **CNS disorders**, or high doses. - Its high affinity for **GABA-A receptors** in the central nervous system is thought to contribute to its proconvulsant effects. *Ertapenem* - While all carbapenems carry some risk of seizures, **ertapenem** has a **lower incidence** compared to imipenem. - It is often favored in patients without CNS infections or severe renal dysfunction due to its once-daily dosing. *Doripenem* - **Doripenem** also has a relatively **low risk of seizures** compared to imipenem. - It is generally well-tolerated, with side effects similar to other carbapenems but at a reduced frequency for CNS events. *Meropenem* - **Meropenem** is known to have a **lower seizure potential** than imipenem, making it a preferred choice for patients with a history of seizures or those with CNS infections. - Its **reduced affinity** for GABA-A receptors contributes to its better CNS tolerability.
Question 10: What is the commonly used concentration of tetracaine for topical anesthesia in minor ophthalmic procedures?
- A. 1%
- B. 0.5% (Correct Answer)
- C. 2%
- D. 0.25%
Explanation: ***0.5%*** - **Tetracaine 0.5%** is the standard and most commonly used concentration for **topical ocular anesthesia** in minor ophthalmic procedures. - This concentration provides effective and rapid onset topical anesthesia for procedures like tonometry, foreign body removal, and gonioscopy with minimal side effects. *1%* - **Tetracaine 1%** is a higher concentration not typically used for routine topical ophthalmic anesthesia due to an increased risk of **epithelial toxicity** and other side effects. - While it would provide more potent anesthesia, its use is generally limited to specific cases where stronger anesthesia is needed and the benefits outweigh the risks. *2%* - **Tetracaine 2%** is an even higher concentration, rarely used in ophthalmology because of a significantly increased risk of **corneal damage** and other ocular surface complications. - This concentration is considered too strong for topical use in the eye and could lead to prolonged epithelial defects. *0.25%* - **Tetracaine 0.25%** is a lower concentration that may not provide sufficient depth or duration of anesthesia for most minor ophthalmic procedures. - While it would have a lower risk of toxicity, its **suboptimal anesthetic effect** makes it less commonly used than 0.5%.
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