Antimicrobial Agents — MCQs

Antimicrobial Agents Indian Medical PG Practice Questions and MCQs

Question 1491: Topical antifungal of choice for Aspergillus infection of the eye?

A. Clotrimazole
B. Miconazole
C. Fluconazole
D. Natamycin (Correct Answer)

Explanation: ***Natamycin*** - **Natamycin (Natacyn) 5%** is the **only FDA-approved topical antifungal** for ophthalmic use and is the **drug of choice** for fungal keratitis caused by filamentous fungi, including *Aspergillus*. - It is a **polyene antifungal** with excellent activity against *Aspergillus* species and has **good corneal penetration** when applied topically. - Natamycin is considered the **gold standard** for treating *Aspergillus* keratitis and is the **first-line agent** for this condition. *Fluconazole* - **Fluconazole** is a triazole antifungal with **poor activity against *Aspergillus* species**. - It is primarily effective against yeasts like *Candida* species, not filamentous fungi. - Fluconazole has **limited role** in treating *Aspergillus* eye infections and is NOT used as a topical agent for this indication. *Miconazole* - **Miconazole** is an imidazole antifungal with limited use in ophthalmic infections. - It has **poor ocular penetration** and is not considered a first-line agent for *Aspergillus* keratitis. *Clotrimazole* - **Clotrimazole** is primarily used for cutaneous and superficial fungal infections. - It has **limited systemic absorption and poor ocular penetration**, making it unsuitable for *Aspergillus* eye infections. - It is not routinely used as a topical ophthalmic antifungal agent.

Question 1492: Which is the fastest acting anti-malarial drug?

A. Chloroquine
B. Quinine
C. Mefloquine
D. Artether (Correct Answer)

Explanation: ***Artether*** - **Artether** is an artemisinin derivative known for its rapid action due to its ability to quickly reduce **parasite biomass**. - It works by producing **free radicals** that damage parasite proteins and membranes, leading to rapid parasitic clearance. *Chloroquine* - **Chloroquine** is a 4-aminoquinoline that works by inhibiting **heme detoxification** in the parasite, but its action is slower compared to artemisinins. - Due to widespread resistance, its efficacy is limited in many regions. *Quinine* - **Quinine** is an alkaloid with proven efficacy and acts by interfering with the parasite's **heme polymerization**. - While effective, its onset of action is not as rapid as that of artemisinin derivatives. *Mefloquine* - **Mefloquine** is a quinoline-methanol derivative used for malaria treatment and prophylaxis. - It has a long half-life and is effective, but its action is slower and it is not considered a rapid-acting drug.

Question 1493: Ivermectin is the drug of choice for which of the following infections?

A. Trichuriasis (whipworm infection)
B. Loiasis (African eye worm)
C. Trichinosis (pork roundworm infection)
D. Onchocerciasis (river blindness caused by Onchocerca volvulus) (Correct Answer)

Explanation: ***Onchocerciasis (river blindness caused by Onchocerca volvulus)*** - **Ivermectin** is the drug of choice for **onchocerciasis** due to its microfilaricidal activity and effectiveness in preventing eye lesions and blindness. - It works by paralyzing and killing **microfilariae**, reducing severe symptoms and transmission. *Trichuriasis (whipworm infection)* - While ivermectin can be used, **albendazole** or **mebendazole** are generally considered the drugs of choice for **Trichuris trichiura** infections. - Ivermectin typically has lower efficacy against adult **Trichuris** worms compared to other anthelmintics. *Loiasis (African eye worm)* - **Diethylcarbamazine (DEC)** is the drug of choice for treating **Loa loa** infection, although ivermectin can be used with caution. - Ivermectin can lead to severe adverse reactions, including potentially fatal **encephalopathy**, in patients with high microfilarial loads of **Loa loa**. *Trichinosis (pork roundworm infection)* - **Albendazole** or **mebendazole** are the primary treatments for **trichinosis**, targeting intestinal adult worms and larvae. - **Ivermectin** is not an approved or recommended treatment for **Trichinella** infections.

Question 1494: Which macrolide is MOST active against Mycobacterium leprae?

A. Erythromycin
B. Azithromycin
C. Roxithromycin
D. Clarithromycin (Correct Answer)

Explanation: **Correct: Clarithromycin** - **Clarithromycin** is the most active macrolide against **Mycobacterium leprae**, with significant in-vitro and in-vivo bactericidal activity - It is a preferred macrolide for leprosy treatment, particularly useful as an alternative in multidrug therapy (MDT) regimens - Its efficacy is attributed to its ability to attain **high intracellular concentrations** and effectively inhibit mycobacterial protein synthesis - Superior to other macrolides in terms of potency against *M. leprae* *Incorrect: Azithromycin* - While **azithromycin** possesses some activity against *M. leprae*, its potency is **less than clarithromycin** - It has a longer half-life and good tissue penetration, but is not the most active macrolide for this indication - Sometimes used in leprosy regimens, but not typically cited as the most active macrolide *Incorrect: Roxithromycin* - **Roxithromycin** has demonstrated some activity against *M. leprae* in laboratory settings, but is **not as extensively studied or commonly used** as clarithromycin for leprosy - Its clinical utility for leprosy is less well-established - Not considered first-line among macrolides for this indication *Incorrect: Erythromycin* - **Erythromycin**, the first macrolide discovered, has **limited activity** against *M. leprae* compared to newer macrolides like clarithromycin - Poor bioavailability and more frequent dosing requirements make it less practical - Not considered a primary agent for the treatment of leprosy

Question 1495: What are the active ingredients in Savlon antiseptic?

A. Cetrimide + Chlorhexidine (Correct Answer)
B. Cetrimide + Chlorhexidine + butyl alcohol
C. Cetrimide + butyl alcohol
D. Cetrimide + Cetavlon

Explanation: ***Cetrimide + Chlorhexidine*** - Savlon antiseptic liquid contains **cetrimide (0.5%)** and **chlorhexidine gluconate (0.1%)** as its active ingredients - This combination provides **broad-spectrum antiseptic activity** with complementary mechanisms - **Cetrimide** acts as a quaternary ammonium compound with surfactant and detergent properties, disrupting bacterial cell membranes - **Chlorhexidine** provides prolonged antimicrobial action by binding to bacterial cell walls and disrupting membrane integrity - This is the **standard formulation** used in Savlon antiseptic liquid *Cetrimide + Chlorhexidine + butyl alcohol* - While cetrimide and chlorhexidine are the active ingredients, **butyl alcohol is not listed as a primary active antiseptic ingredient** in standard Savlon formulation - Butyl alcohol may be present as a **solvent or excipient** but not as an active antiseptic agent *Cetrimide + butyl alcohol* - This combination **omits chlorhexidine**, which is a crucial component for Savlon's antiseptic efficacy - Butyl alcohol is not an active antiseptic ingredient in Savlon *Cetrimide + Cetavlon* - **Cetavlon is a trade name** for preparations containing cetrimide itself, not a separate active ingredient - This option represents a **redundancy** as it essentially lists cetrimide twice under different names

Question 1496: Mechanism of action of Linezolid is

A. Inhibits 30S ribosomal subunit
B. Inhibits 23S ribosome subunit of 50S ribosome
C. Inhibits peptide bond formation (Correct Answer)
D. Inhibits tRNA translocation on the ribosome

Explanation: ***Inhibits peptide bond formation*** - Linezolid is an **oxazolidinone antibiotic** that acts at the **initiation stage of bacterial protein synthesis**. - It binds to the **23S ribosomal RNA** of the 50S subunit, preventing formation of the **70S initiation complex** and thereby **blocking the formation of the first peptide bond**. - This is the **functional mechanism** that defines its antibacterial action. *Inhibits 30S ribosomal subunit* - This mechanism is characteristic of **aminoglycosides** (e.g., gentamicin) and **tetracyclines**, which target the 30S ribosomal subunit. - Linezolid specifically targets the **50S subunit**, not the 30S subunit. *Inhibits 23S ribosome subunit of 50S ribosome* - While this correctly describes Linezolid's **binding site** (23S rRNA of the 50S subunit), it does not specify the **functional consequence** of this binding. - The **mechanism of action** in pharmacology refers to the functional effect, which is **inhibition of peptide bond formation** at the initiation stage. - This option describes "where" the drug acts rather than "what" it does at that site. *Inhibits tRNA translocation on the ribosome* - This is the mechanism of **macrolides** (e.g., erythromycin, azithromycin) and **chloramphenicol**, which block the translocation of peptidyl-tRNA from the A-site to the P-site. - Linezolid acts at the **initiation phase**, not during elongation/translocation.

Question 1497: Drug of choice for surgical prophylaxis?

A. Cefaclor
B. Ceftizoxime
C. Cefazolin (Correct Answer)
D. Cefoperazone

Explanation: ***Cefazolin*** - **Cefazolin** is a first-generation cephalosporin that provides excellent coverage against **Staphylococcus aureus** and **coagulase-negative staphylococci**, which are common surgical site pathogens. - Its **longer half-life** allows for less frequent dosing and it achieves good tissue penetration, making it ideal for surgical prophylaxis. *Cefaclor* - **Cefaclor** is a second-generation cephalosporin with a broader spectrum of activity but a **shorter half-life**, which makes it less suitable for sustained prophylactic levels. - It offers good oral bioavailability but is generally **not preferred for intravenous surgical prophylaxis** compared to cefazolin. *Ceftizoxime* - **Ceftizoxime** is a third-generation cephalosporin with **broader gram-negative coverage** than cefazolin, but this is often not necessary for routine surgical prophylaxis. - Its spectrum includes organisms less commonly implicated in surgical site infections, and its use could **contribute to antibiotic resistance** without added benefit. *Cefoperazone* - **Cefoperazone** is a third-generation cephalosporin with good activity against **Pseudomonas aeruginosa** and gram-negative bacteria, often used for more serious or nosocomial infections. - Its **broader spectrum** and higher cost make it less appropriate for standard surgical prophylaxis, where a narrower-spectrum agent like cefazolin is effective and preferred.

Question 1498: For systemic mycosis, fluconazole is preferred over ketoconazole because of:

A. Lesser side effects
B. All of the options (Correct Answer)
C. Greater efficacy
D. Longer t1/2

Explanation: ***All of the options*** Fluconazole is preferred over ketoconazole for systemic mycoses due to multiple superior pharmacological properties: **Greater Efficacy:** - Fluconazole has **excellent bioavailability** (>90% oral) and superior tissue penetration, especially into the CNS and CSF - Highly effective against **Candida species** (including systemic candidiasis) and **Cryptococcus neoformans** (cryptococcal meningitis) - Ketoconazole has poor CNS penetration, making it unsuitable for deep-seated infections **Longer Half-life (t1/2):** - Fluconazole has a **half-life of ~30 hours**, enabling **once-daily dosing** - Ketoconazole has a half-life of ~8 hours, requiring multiple daily doses - Longer half-life improves patient compliance and maintains therapeutic drug levels **Lesser Side Effects:** - Fluconazole has **minimal hepatotoxicity** and is generally well-tolerated - Ketoconazole causes **significant hepatotoxicity** (black box warning) and **endocrine disturbances** by blocking steroid synthesis (gynecomastia, decreased testosterone, adrenal suppression) - Fluconazole does not interfere with steroid hormone synthesis at therapeutic doses **Clinical Practice:** Due to these combined advantages, ketoconazole is now rarely used for systemic mycoses and is primarily reserved for topical applications or specific resistant dermatophyte infections.

Question 1499: Typhoid is treated by all except

A. Amikacin
B. Ciprofloxacin
C. Ceftriaxone
D. Erythromycin (Correct Answer)

Explanation: ***Erythromycin*** - **Erythromycin** is a macrolide antibiotic primarily effective against Gram-positive bacteria and atypical bacteria. - It has **NO role** in the treatment of typhoid fever as it lacks adequate activity against *Salmonella typhi* and does not achieve therapeutic intracellular concentrations. - Unlike azithromycin (another macrolide), erythromycin has poor intracellular penetration and is **never used** for typhoid. *Ceftriaxone* - **Ceftriaxone** is a third-generation cephalosporin that is highly effective against *Salmonella typhi*. - It is a **first-line treatment** for severe typhoid fever and is preferred in regions with fluoroquinolone resistance. - Dosage: 2-4 g IV daily for 10-14 days. *Amikacin* - **Amikacin** is an aminoglycoside with generally poor intracellular penetration, making it suboptimal for treating intracellular pathogens. - While **not a standard agent** for typhoid, it has been used in **rare cases of extensively drug-resistant (XDR) typhoid** as part of combination therapy, particularly in outbreaks where other options have failed. - It is **not a recommended first-line or routine agent** due to limited efficacy, potential toxicity (nephrotoxicity and ototoxicity), and availability of better alternatives. *Ciprofloxacin* - **Ciprofloxacin** is a fluoroquinolone with excellent intracellular penetration and was historically the **first-line oral treatment** for uncomplicated typhoid fever. - Its efficacy against *Salmonella typhi* made it the preferred agent for decades. - However, widespread **fluoroquinolone resistance** has emerged in South Asia, limiting its current use as empirical first-line therapy in many regions.

Question 1500: Which of these is least effective for the treatment of typhoid?

A. Ceftriaxone
B. Chloramphenicol
C. Cefixime
D. Ciprofloxacin (Correct Answer)

Explanation: ***Ciprofloxacin*** - While ciprofloxacin was historically a first-line treatment, increasing rates of **quinolone resistance** among *Salmonella Typhi* isolates in many regions have significantly reduced its effectiveness. - The presence of **mutations** in the *gyrA* gene reduces the susceptibility of the bacteria to fluoroquinolones, making it a less reliable empirical treatment. *Chloramphenicol* - **Chloramphenicol** was one of the earliest effective antibiotics for typhoid fever and remains active against many strains, particularly in regions where resistance has not extensively developed. - Its use is limited by potential severe side effects such as **bone marrow suppression** (aplastic anemia), which makes it a less preferred option when safer alternatives are available. *Ceftriaxone* - **Ceftriaxone**, a third-generation cephalosporin, is often used for the treatment of severe or multidrug-resistant (MDR) typhoid fever due to its good activity against *Salmonella Typhi*. - It is typically administered parenterally, making it suitable for hospitalized patients with serious infections, and boasts a favorable **safety profile**. *Cefixime* - **Cefixime** is an oral third-generation cephalosporin that is effective against *Salmonella Typhi* and is often used for outpatient treatment of uncomplicated typhoid fever. - Its primary advantage is its **oral bioavailability**, which allows for step-down therapy after initial parenteral treatment or as a primary treatment in less severe cases.

Practice by Chapter

Beta-Lactam Antibiotics

Practice Questions

Aminoglycosides

Practice Questions

Macrolides and Ketolides

Practice Questions

Tetracyclines

Practice Questions

Quinolones

Practice Questions

Sulfonamides and Trimethoprim

Practice Questions

Antimycobacterial Drugs

Practice Questions

Antifungal Agents

Practice Questions

Antiviral Drugs

Practice Questions

Antiparasitic Agents

Practice Questions

Principles of Antimicrobial Selection

Practice Questions

Antimicrobial Resistance

Practice Questions

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