Antimicrobial Agents — MCQs

Antimicrobial Agents Indian Medical PG Practice Questions and MCQs

Question 221: Select the drug that is active against both HIV and hepatitis B virus?

A. Lamivudine (Correct Answer)
B. Indinavir
C. Didanosine
D. Efavirenz

Explanation: **Explanation:** **Lamivudine (3TC)** is a nucleoside reverse transcriptase inhibitor (NRTI) that inhibits the reverse transcriptase enzyme of HIV. Crucially, it also inhibits the **HBV DNA polymerase**, making it effective against both viruses. This dual activity is a high-yield concept in NEET-PG, as these drugs are preferred in patients with HIV/HBV co-infection. **Analysis of Options:** * **Lamivudine (Correct):** An NRTI used in HAART (Highly Active Antiretroviral Therapy) and for chronic Hepatitis B. Other drugs with this dual activity include **Tenofovir (TDF/TAF)** and **Emtricitabine (FTC)**. * **Indinavir (Incorrect):** This is a **Protease Inhibitor (PI)**. While effective against HIV, it has no activity against the HBV DNA polymerase. * **Didanosine (Incorrect):** An older NRTI used for HIV. Unlike Lamivudine, it does not possess significant clinical activity against HBV. * **Efavirenz (Incorrect):** A Non-Nucleoside Reverse Transcriptase Inhibitor (NNRTI). It is specific to HIV-1 and does not affect HBV. **High-Yield Clinical Pearls for NEET-PG:** 1. **Dual-Action Drugs:** Remember the mnemonic **"LET"** for drugs active against both HIV and HBV: **L**amivudine, **E**mtricitabine, and **T**enofovir. 2. **Withdrawal Flare:** If Lamivudine or Tenofovir is discontinued in a co-infected patient, it can lead to a severe, life-threatening rebound flare of Hepatitis B. 3. **Resistance:** Lamivudine has a low genetic barrier; HBV resistance (M204V mutation) develops rapidly if used as monotherapy.

Question 222: The dose of which of the following antibiotics does not require alteration in renal failure?

A. Vancomycin
B. Ethambutol
C. Erythromycin (Correct Answer)
D. Metronidazole

Explanation: **Explanation:** The primary factor determining whether an antibiotic dose requires adjustment in renal failure is its **route of elimination**. Drugs primarily excreted via the kidneys require dose reduction to prevent toxicity, whereas drugs metabolized by the liver or excreted via bile do not. **Correct Option: C. Erythromycin** Erythromycin is a macrolide antibiotic primarily metabolized by the liver and excreted in the **bile**. Since its clearance is independent of renal function, no dose adjustment is necessary in patients with renal impairment. **Incorrect Options:** * **A. Vancomycin:** This glycopeptide is almost entirely excreted unchanged by glomerular filtration. It is highly nephrotoxic; thus, dosing must be strictly adjusted based on creatinine clearance or serum drug monitoring. * **B. Ethambutol:** Approximately 80% of this antitubercular drug is excreted unchanged in the urine. Accumulation in renal failure can lead to **optic neuritis**. * **C. Metronidazole:** While primarily metabolized by the liver, its metabolites are renally excreted. In end-stage renal disease (ESRD), a dose reduction (usually 50%) is often recommended to prevent the accumulation of active metabolites. **High-Yield Clinical Pearls for NEET-PG:** * **"Safe" Antibiotics in Renal Failure (No/Minimal adjustment):** Erythromycin, Azithromycin, Ceftriaxone, Doxycycline, Chloramphenicol, Clindamycin, and Rifampicin. * **Doxycycline** is the tetracycline of choice in renal failure because it is excreted via feces (enterohepatic circulation). * **Ceftriaxone** is a third-generation cephalosporin that does not require adjustment due to dual excretion (biliary and renal).

Question 223: All of the following parasitic infestations are indications for the use of Mebendazole, except?

A. Roundworm
B. Whipworm
C. Hookworm
D. Strongyloides (Correct Answer)

Explanation: **Explanation:** **Mebendazole** is a broad-spectrum anthelmintic belonging to the Benzimidazole group. It works by inhibiting **microtubule synthesis** (binding to β-tubulin) in the parasite, leading to glucose depletion and death. **Why Strongyloides is the Correct Answer:** While Mebendazole has some activity against many nematodes, it is **not** the drug of choice for *Strongyloides stercoralis*. Strongyloidiasis requires drugs that can penetrate deeper into the intestinal mucosa and systemic circulation. The gold standard treatment for *Strongyloides* is **Ivermectin** (first-line) or **Albendazole** (second-line). Mebendazole has poor systemic absorption, making it less effective for this specific parasite. **Analysis of Incorrect Options:** * **A. Roundworm (*Ascaris lumbricoides*):** Mebendazole is highly effective and a standard treatment for ascariasis. * **B. Whipworm (*Trichuris trichiura*):** Mebendazole is considered the drug of choice for whipworm infections, often requiring a 3-day course. * **C. Hookworm (*Ancylostoma duodenale/Necator americanus*):** Mebendazole is a primary treatment option for hookworm infestations. **High-Yield Clinical Pearls for NEET-PG:** 1. **Mechanism of Action:** Inhibition of microtubule polymerization by binding to β-tubulin. 2. **Drug of Choice (DOC) Summary:** * **Ivermectin:** DOC for *Strongyloides* and Onchocerciasis (River blindness). * **Albendazole:** DOC for Hydatid disease (*Echinococcus*) and Neurocysticercosis (*Taenia solium*). * **Diethylcarbamazine (DEC):** DOC for Filariasis. 3. **Contraindication:** Benzimidazoles are generally avoided in the **first trimester of pregnancy** due to potential embryotoxicity.

Question 224: What is the drug of choice for nasal carriers of MRSA?

A. Vancomycin
B. Ceftobipirole
C. Linezolid
D. Mupirocin topical (Correct Answer)

Explanation: **Explanation:** **Mupirocin (Topical)** is the drug of choice for the eradication of Methicillin-resistant *Staphylococcus aureus* (MRSA) in nasal carriers. The primary reservoir for MRSA in humans is the anterior nares. Mupirocin works by inhibiting the enzyme **isoleucyl-tRNA synthetase**, thereby blocking bacterial protein synthesis. Its topical application allows for high local concentrations in the nasal mucosa, effectively decolonizing the patient and preventing the spread of infection without the need for systemic antibiotics. **Analysis of Incorrect Options:** * **Vancomycin:** While it is the systemic gold standard for treating invasive MRSA infections (e.g., endocarditis, pneumonia), it is not used for nasal decolonization due to poor mucosal penetration and the risk of promoting resistance (VRSA). * **Ceftobipirole:** This is a 5th-generation cephalosporin with activity against MRSA. It is reserved for serious systemic infections like community-acquired pneumonia and is not indicated for topical carriage eradication. * **Linezolid:** An oxazolidinone used for systemic MRSA and VRE infections. Using a potent systemic agent for simple nasal colonization is clinically inappropriate and contributes to antibiotic resistance. **High-Yield Clinical Pearls for NEET-PG:** * **Mupirocin** is also the drug of choice for **Impetigo** (caused by *S. aureus* or *S. pyogenes*). * **Decolonization Protocol:** Usually involves applying Mupirocin ointment to the inner nares twice daily for 5 days. * **Alternative:** If Mupirocin resistance occurs, topical **Retapamulin** or **Povidone-iodine** may be used. * **Mechanism Check:** Remember that Mupirocin is unique because it inhibits **tRNA synthetase**, not the ribosome itself.

Question 225: What is the drug of choice for syphilis?

A. Benzathine Penicillin (Correct Answer)
B. Penicillin
C. Tetracycline
D. Ciprofloxacin

Explanation: **Explanation:** **1. Why Benzathine Penicillin is the Correct Answer:** Benzathine Penicillin G is the drug of choice (DOC) for syphilis caused by *Treponema pallidum*. The underlying medical concept is its **pharmacokinetics**: it is a repository (long-acting) form of penicillin. After a single intramuscular injection, it releases penicillin slowly into the bloodstream, maintaining low but effective treponemicidal concentrations for 2–3 weeks. Since *T. pallidum* has a slow multiplication time (30–33 hours), continuous exposure to the antibiotic is essential for eradication. **2. Why Other Options are Incorrect:** * **Option B (Penicillin):** While technically correct regarding the class, "Penicillin" usually refers to Penicillin G (aqueous), which has a very short half-life (30 minutes). It requires frequent dosing and is not practical for outpatient management of primary or secondary syphilis. * **Option C (Tetracycline):** This is a second-line alternative (usually Doxycycline) used only in patients with a documented penicillin allergy. It is less effective and requires a long course (14–28 days), leading to poor compliance. * **Option D (Ciprofloxacin):** Fluoroquinolones have no significant activity against *T. pallidum* and are not used in the treatment of syphilis. **3. High-Yield Clinical Pearls for NEET-PG:** * **Jarisch-Herxheimer Reaction:** A classic boards topic; it is an acute febrile reaction occurring within hours of the first penicillin dose due to the release of endotoxins from dying spirochetes. It is managed with aspirin/NSAIDs. * **Neurosyphilis:** The DOC is **Aqueous Crystalline Penicillin G** (IV), as Benzathine penicillin does not achieve adequate levels in the CSF. * **Pregnancy:** Penicillin is the *only* recommended treatment. If the mother is allergic, she must undergo **desensitization** rather than using alternative drugs like Doxycycline (which is contraindicated). * **Dose:** 2.4 million units IM (single dose for primary/secondary; weekly for 3 weeks for late latent).

Question 226: Time-dependent killing and prolonged post-antibiotic effect is a characteristic of which class of antibiotics?

A. Fluoroquinolones
B. Beta-lactam antibiotics (Correct Answer)
C. Clindamycin
D. Erythromycin

Explanation: ### Explanation **Correct Answer: B. Beta-lactam antibiotics** **Understanding the Concept:** Pharmacodynamics of antibiotics are categorized based on their killing patterns. **Beta-lactams** (Penicillins, Cephalosporins, Carbapenems) exhibit **Time-dependent killing**. This means their efficacy depends on the duration the drug concentration remains above the Minimum Inhibitory Concentration (**T > MIC**) rather than the peak concentration. While most time-dependent drugs have a short **Post-Antibiotic Effect (PAE)** against Gram-negative bacteria, Beta-lactams (especially Carbapenems) demonstrate a **prolonged PAE** against Gram-positive organisms. In clinical practice, this necessitates frequent dosing or continuous infusion to maintain levels above MIC for at least 40-50% of the dosing interval. **Analysis of Incorrect Options:** * **A. Fluoroquinolones:** These exhibit **Concentration-dependent killing** and a prolonged PAE. Their efficacy is determined by the **AUC/MIC** ratio or **Cmax/MIC** ratio. * **C & D. Clindamycin and Erythromycin (Macrolides):** These are primarily bacteriostatic agents. While they are time-dependent, they are characterized by a significant PAE, but they do not fit the classic "Time-dependent killing" profile as strictly as Beta-lactams in the context of this standard pharmacological classification. **High-Yield Clinical Pearls for NEET-PG:** 1. **Concentration-dependent killing (Cmax/MIC):** Aminoglycosides, Fluoroquinolones, Daptomycin, and Metronidazole. 2. **Time-dependent killing (T > MIC):** Beta-lactams, Linezolid, and Vancomycin (though Vancomycin is increasingly linked to AUC/MIC). 3. **Post-Antibiotic Effect (PAE):** This is the persistent suppression of bacterial growth after the drug concentration falls below the MIC. Aminoglycosides have the most significant PAE among concentration-dependent drugs. 4. **Pulse Dosing:** Used for Aminoglycosides to exploit their concentration-dependent killing and prolonged PAE, allowing for once-daily dosing while minimizing nephrotoxicity.

Question 227: Ganciclovir is used in the treatment of which of the following viral infections?

A. Adenovirus
B. Cytomegalovirus (Correct Answer)
C. Epstein-Barr virus
D. Arenavirus

Explanation: **Explanation:** **Ganciclovir** is a synthetic analogue of 2'-deoxyguanosine and is the **drug of choice** for infections caused by **Cytomegalovirus (CMV)**. ### 1. Why Cytomegalovirus (CMV) is Correct: Ganciclovir’s mechanism of action is highly specific to CMV. Inside a CMV-infected cell, the viral protein kinase **UL97** catalyzes the initial phosphorylation of Ganciclovir to Ganciclovir-monophosphate. Host cell enzymes then convert it into the triphosphate form, which competitively inhibits **viral DNA polymerase** and terminates DNA chain elongation. Because it requires the specific UL97 kinase for activation, it is significantly more potent against CMV than Acyclovir. ### 2. Why Other Options are Incorrect: * **Adenovirus:** While some severe cases are treated with Cidofovir or Brincidofovir, Ganciclovir is not the standard treatment. * **Epstein-Barr virus (EBV):** Although Ganciclovir has some *in vitro* activity against EBV, it is rarely used clinically for EBV as the virus lacks the specific kinases required for efficient drug activation. * **Arenavirus:** This family (e.g., Lassa fever) consists of RNA viruses. Ganciclovir only targets DNA viruses. **Ribavirin** is the drug of choice for Arenaviruses. ### 3. High-Yield Clinical Pearls for NEET-PG: * **Major Side Effect:** The dose-limiting toxicity of Ganciclovir is **Bone Marrow Suppression** (specifically neutropenia and thrombocytopenia). * **Valganciclovir:** This is the L-valyl ester prodrug of Ganciclovir with high oral bioavailability, used for CMV prophylaxis and maintenance. * **Resistance:** Resistance to Ganciclovir occurs due to mutations in the **UL97 gene** (preventing phosphorylation) or the **UL54 gene** (mutating DNA polymerase). * **Foscarnet:** Used as an alternative in Ganciclovir-resistant CMV; it does not require viral phosphorylation for its action.

Question 228: Which bacterium is primarily implicated in the development of dental caries?

A. Streptococcus mutans (Correct Answer)
B. Pneumococci
C. Streptococcus pyogenes
D. Staphylococcus aureus

Explanation: **Explanation:** **Streptococcus mutans** is the primary etiologic agent of dental caries. The underlying medical concept involves its unique ability to metabolize dietary sucrose into **extracellular polysaccharides (glucans)** via the enzyme glucosyltransferase. These glucans allow the bacteria to adhere firmly to the tooth enamel, forming a biofilm known as **dental plaque**. Once attached, *S. mutans* ferments carbohydrates to produce **lactic acid**, which lowers the local pH below 5.5, leading to the demineralization of the tooth enamel. **Analysis of Incorrect Options:** * **Pneumococci (*S. pneumoniae*):** These are alpha-hemolytic, bile-soluble, capsulated cocci primarily responsible for community-acquired pneumonia, meningitis, and otitis media. They do not colonize the dental pellicle. * **Streptococcus pyogenes (Group A Strep):** This is a major human pathogen causing pharyngitis, impetigo, and post-streptococcal sequelae like Rheumatic Fever. It is not associated with tooth decay. * **Staphylococcus aureus:** A catalase-positive, coagulase-positive organism known for causing skin infections, abscesses, and osteomyelitis. While it can be found in the oral cavity, it is not the initiator of dental caries. **High-Yield Clinical Pearls for NEET-PG:** * **Viridans Group Streptococci:** *S. mutans* belongs to this group. Remember that *S. sanguinis* is also found in plaque but *S. mutans* is the most cariogenic. * **Infective Endocarditis:** If *S. mutans* or *S. mitis* enters the bloodstream (e.g., during dental procedures), they can cause subacute bacterial endocarditis (SBE) on damaged heart valves. * **Lactobacilli:** While *S. mutans* initiates the cavity, *Lactobacillus* species are often responsible for the progression of deep dentinal caries.

Question 229: Tetracycline acts on which part of the bacterial cell?

A. 30S ribosomes (Correct Answer)
B. 50S ribosomes
C. Cell membrane
D. Inhibiting DNA gyrase

Explanation: **Explanation:** **Mechanism of Action (Correct Answer: A):** Tetracyclines are **bacteriostatic** antibiotics that inhibit bacterial protein synthesis. They cross the bacterial cell wall via passive diffusion and active transport. Once inside, they bind reversibly to the **30S ribosomal subunit**. Specifically, they block the attachment of aminoacyl-tRNA to the 'A' site on the mRNA-ribosome complex, preventing the addition of new amino acids to the growing peptide chain. **Analysis of Incorrect Options:** * **B. 50S ribosomes:** This is the site of action for Macrolides (Erythromycin, Azithromycin), Chloramphenicol, Clindamycin, and Linezolid. * **C. Cell membrane:** Drugs like Polymyxins (Polymyxin B, Colistin) and Daptomycin act by disrupting the integrity of the bacterial cell membrane. * **D. Inhibiting DNA gyrase:** This is the mechanism of Fluoroquinolones (e.g., Ciprofloxacin, Levofloxacin), which inhibit DNA replication by targeting DNA gyrase (Topoisomerase II) and Topoisomerase IV. **NEET-PG High-Yield Pearls:** 1. **Resistance:** Primarily occurs via **efflux pumps** (encoded by the *tet* gene), which pump the drug out of the cell. 2. **Spectrum:** Broad-spectrum; drug of choice for **Rickettsial infections**, Chlamydia, Cholera, and Brucellosis. 3. **Contraindications:** Avoided in pregnancy and children under 8 years due to **chelation with calcium**, leading to permanent tooth discoloration and inhibition of bone growth. 4. **Specific Drugs:** **Doxycycline** is the only tetracycline safe in renal failure (excreted via bile) and is the drug of choice for Lyme disease. **Tigecycline** (a glycylcycline) is designed to overcome efflux-mediated resistance.

Question 230: Which of the following drugs should not be used to treat Klebsiella infection?

A. Ampicillin (Correct Answer)
B. Amikacin
C. Imipenem
D. Tigecycline

Explanation: ### Explanation **1. Why Ampicillin is the Correct Answer:** The primary reason **Ampicillin** is ineffective against *Klebsiella pneumoniae* is **intrinsic resistance**. *Klebsiella* species possess chromosomally mediated beta-lactamases (specifically SHV-1) that hydrolyze the penicillin ring. Therefore, *Klebsiella* is naturally resistant to ampicillin, amoxicillin, and ticarcillin. Using these drugs would result in clinical failure. **2. Analysis of Incorrect Options:** * **Amikacin (Option B):** This is an aminoglycoside often used in combination therapy for serious Gram-negative infections, including *Klebsiella*. It remains effective unless the strain produces aminoglycoside-modifying enzymes. * **Imipenem (Option C):** A carbapenem that is traditionally the "drug of choice" for Extended-Spectrum Beta-Lactamase (ESBL) producing *Klebsiella*. While Carbapenem-Resistant Enterobacteriaceae (CRE) are emerging, Imipenem remains a standard treatment option. * **Tigecycline (Option D):** A glycylcycline used as a reserve drug for multi-drug resistant (MDR) *Klebsiella* and CRE infections, particularly in complicated skin or intra-abdominal infections. **3. High-Yield Clinical Pearls for NEET-PG:** * **Intrinsic Resistance:** Always remember "KEE" organisms (Klebsiella, Enterobacter, and Enterococci) have specific intrinsic resistance patterns. *Klebsiella* = Ampicillin resistance. * **Drug of Choice:** For ESBL-producing *Klebsiella*, **Carbapenems** (Imipenem/Meropenem) are the preferred treatment. * **Newer Agents:** For Carbapenem-resistant strains, newer combinations like **Ceftazidime-Avibactam** or older drugs like **Colistin** and **Polymyxin B** are utilized. * **Morphology:** *Klebsiella* is a Gram-negative, encapsulated, non-motile bacilli known for producing "mucoid" colonies on culture.

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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