Antimicrobial Agents — MCQs

Antimicrobial Agents Indian Medical PG Practice Questions and MCQs

Question 1491: 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 1492: Which of the following statements about the primary clinical use of levamisole is true?

A. It has immunomodulator action
B. It is the levoisomer of tetramisole
C. It can kill strongyloides larvae and adult worms
D. It is used against ascariasis and ancyclostomiasis (Correct Answer)

Explanation: ***It is used against ascariasis and ancyclostomiasis*** - **Levamisole** is an **anthelmintic drug** primarily used for the treatment of **nematode infections**, specifically **roundworms (Ascaris lumbricoides)** and **hookworms (Ancylostoma duodenale)**. - It works by **paralyzing the worms**, leading to their expulsion in the feces. *It is the levoisomer of tetramisole* - While it is true that **levamisole** is the **levorotatory isomer** of **tetramisole**, this statement describes its chemical structure, not its primary clinical use. - **Tetramisole** itself exists as a **racemic mixture** of both levo and dextro isomers. *It has immunomodulator action* - **Levamisole** does possess **immunomodulatory properties** at lower doses and was historically investigated for conditions like **rheumatoid arthritis** and as an adjuvant in colon cancer. - However, its primary and most widespread clinical use is as an **anthelmintic**, not as an immunomodulator, especially due to concerns about side effects like **agranulocytosis**. *It can kill strongyloides larvae and adult worms* - **Levamisole** is **not effective** against **Strongyloides stercoralis** (threadworms). - The drug of choice for **strongyloidiasis** is typically **ivermectin** or **albendazole**.

Question 1493: 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 1494: 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 1495: 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 1496: 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 1497: 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 1498: Which of the following is a topical antifungal agent?

A. Benzyl benzoate
B. Brimetenide
C. Butenafine (Correct Answer)
D. Posaconazole

Explanation: ***Butenafine*** - **Butenafine** belongs to the class of **benzylamine antifungals** and is used topically to treat fungal infections such as **tinea pedis (athlete's foot)**, **tinea cruris (jock itch)**, and **tinea corporis (ringworm)**. - Its mechanism of action involves inhibiting **squalene epoxidase**, an enzyme essential for fungal cell membrane synthesis, leading to **ergosterol depletion** and accumulation of squalene, which is toxic to the fungus. *Benzyl benzoate* - **Benzyl benzoate** is primarily used as a **topical scabicide** and **pediculicide**, meaning it targets mites (scabies) and lice. - It works by acting as a **neurotoxin** to these parasites, causing paralysis and death, and is not classified as an antifungal agent. *Brimetenide* - This option appears to be a **misspelling** or **non-existent drug name** in common pharmaceutical contexts. - There is no widely recognized drug with the name **"Brimetenide"** that serves as an antifungal. *Posaconazole* - **Posaconazole** is a **systemic triazole antifungal**, typically administered orally or intravenously for the treatment and prophylaxis of invasive fungal infections in immunocompromised patients. - While it is an antifungal, its primary use is **systemic** rather than topical for treating superficial 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: Colistin is obtained from ?

A. Bacteria (Correct Answer)
B. Fungi
C. Herbs
D. Actinomycetes

Explanation: ***Bacteria*** - Colistin, also known as **polymyxin E**, is an antibiotic derived from the bacterium **Bacillus polymyxa subspecies colistinus** (now reclassified as *Paenibacillus polymyxa*). - It works by disrupting the **outer membrane** of Gram-negative bacteria, making it effective against multi-drug resistant strains. - While actinomycetes are also bacteria, in pharmacology, antibiotics from **Bacillus species** are categorized separately from actinomycete-derived drugs. *Fungi* - Many common antibiotics, such as **penicillin** and **cephalosporins**, are derived from fungi (e.g., *Penicillium*, *Cephalosporium*), but colistin is not one of them. - Fungal-derived antibiotics typically target bacterial **cell wall synthesis** or **protein synthesis**. *Actinomycetes* - **Actinomycetes**, particularly from the *Streptomyces* genus, are a rich source of antibiotics, including **streptomycin**, **tetracyclines**, and **erythromycin**. - While actinomycetes are technically bacteria, they are classified separately in pharmacology due to their unique characteristics and prolific antibiotic production. - Colistin is specifically produced by **Bacillus species**, not actinomycetes. *Herbs* - Herbs are used for traditional medicine and can have various therapeutic properties, but they do **not produce antibiotics** like colistin. - While some plant-derived compounds have antimicrobial activity, colistin is a product of **microbial fermentation**.

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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