Antimicrobial Agents — MCQs

Antimicrobial Agents Indian Medical PG Practice Questions and MCQs

Question 1551: What is the drug of choice for listeria meningitis?

A. Ampicillin (Correct Answer)
B. Cefotaxime
C. Ciprofloxacin
D. Ceftriaxone

Explanation: ***Ampicillin*** - **Ampicillin** is the **drug of choice** for *Listeria monocytogenes* meningitis due to its excellent in vitro activity and good central nervous system penetration. - It is often used in combination with an **aminoglycoside** (e.g., gentamicin) for synergistic bactericidal activity, especially in severe cases, though gentamicin does not penetrate the CSF well. *Cefotaxime* - **Third-generation cephalosporins** like cefotaxime have poor activity against *Listeria monocytogenes* due to the organism's intrinsic resistance to these agents. - While effective against many other bacterial causes of meningitis (e.g., *S. pneumoniae*, *N. meningitidis*), it is not appropriate for *Listeria*. *Ceftriaxone* - Similar to cefotaxime, **ceftriaxone** is a third-generation cephalosporin and is **ineffective** against *Listeria monocytogenes* due to the lack of penicillin-binding protein (PBP) affinity. - Its use for *Listeria* meningitis would lead to treatment failure. *Ciprofloxacin* - **Ciprofloxacin**, a fluoroquinolone, is generally **not recommended** as a first-line treatment for *Listeria* meningitis, despite some in vitro activity. - Its use is typically reserved for patients with severe allergies to penicillins, and even then, **trimethoprim-sulfamethoxazole** is usually preferred as an alternative to ampicillin.

Question 1552: What is the best skin disinfectant for central line insertion?

A. Alcohol
B. Cetrimide
C. Chlorhexidine (Correct Answer)
D. Povidone iodine

Explanation: ***Chlorhexidine*** - **Chlorhexidine (particularly >0.5% chlorhexidine in alcohol-based solution, such as 2% chlorhexidine in 70% isopropyl alcohol)** is the preferred antiseptic for central line insertion per **CDC guidelines**. - It provides **rapid onset of action**, persistent antimicrobial activity (lasting several hours), and broad-spectrum efficacy against gram-positive and gram-negative bacteria, fungi, and some viruses. - Superior to povidone-iodine in reducing catheter-related bloodstream infections (CRBSIs) in multiple studies. - Its mechanism involves disrupting bacterial cell membranes and coagulating intracellular contents, leading to sustained antimicrobial activity on the skin. *Povidone iodine* - **Povidone iodine** has a slower onset of action and is inactivated by organic matter (blood, serum), making it less effective for immediate, sustained disinfection compared to chlorhexidine. - While it has broad-spectrum activity, its residual effect is limited once it dries on the skin. - Studies show higher rates of catheter-related infections compared to chlorhexidine-based antiseptics. *Alcohol* - **Alcohol** (e.g., isopropyl alcohol or ethanol) provides good immediate microbial kill but lacks persistent activity, meaning its effect is short-lived as it evaporates quickly from the skin. - It works by denaturing proteins and dissolving lipids, but its rapid evaporation makes it insufficient as a sole agent for central line insertion. - Often used as a component in combination with chlorhexidine for optimal efficacy. *Cetrimide* - **Cetrimide** is a quaternary ammonium compound with antiseptic properties, but it has a narrower spectrum of activity and is less potent than chlorhexidine for surgical site preparation. - It is often used in combination with other agents or for general skin cleansing rather than for critical procedures like central line insertion. - Not recommended as a primary antiseptic for central venous catheter insertion.

Question 1553: Which sulphonamide has the longest acting duration?

A. Sulfadiazine
B. Sulphadoxine (Correct Answer)
C. Sulfamethoxazole
D. Sulfamethiazole

Explanation: ***Sulphadoxine*** - **Sulphadoxine** is known for its **exceptionally long elimination half-life**, which is due to its high plasma protein binding and slow renal excretion. - This property allows for **once-weekly dosing**, making it one of the longest-acting sulfonamides, often used in combinations for malaria prophylaxis or treatment. *Sulfadiazine* - **Sulfadiazine** has an intermediate half-life, typically requiring **multiple daily doses**. - It is commonly used for infections like **toxoplasmosis** and **nocardiosis**. *Sulfamethoxazole* - **Sulfamethoxazole** has an intermediate half-life, usually requiring **twice-daily administration**. - It is most famously co-formulated with **trimethoprim** (as co-trimoxazole) for a broad range of bacterial infections. *Sulfamethiazole* - **Sulfamethiazole** is a **short-acting sulfanilamide derivative** with a rapid elimination, meaning it would require frequent dosing. - It is not commonly used systemically due to its short duration of action.

Question 1554: Which type of vaccine is used for chicken pox?

A. Live vaccine (Correct Answer)
B. Killed vaccine
C. Conjugated vaccine
D. Toxoid vaccine

Explanation: ***Live vaccine*** - The chickenpox vaccine (Varicella vaccine) is a **live-attenuated vaccine**, meaning it contains a weakened form of the **Varicella-zoster virus** (Oka strain). - This weakened virus can still replicate in the body, stimulating a strong and long-lasting immune response similar to natural infection but without causing severe disease. *Killed vaccine* - **Killed (inactivated) vaccines** use viruses or bacteria that have been inactivated through heat or chemicals, making them unable to replicate. - While effective for some diseases (e.g., inactivated polio, influenza), they typically require **multiple doses** and might provide less durable immunity compared to live vaccines. *Conjugated vaccine* - **Conjugated vaccines** are designed to improve the immune response to polysaccharide antigens (e.g., bacterial capsules) by linking them to a carrier protein. - This technology is primarily used for **bacterial infections** like *Haemophilus influenzae* type b (Hib) or pneumococcal disease, not viral illnesses like chickenpox. *Toxoid vaccine* - **Toxoid vaccines** contain inactivated bacterial toxins (toxoids) rather than the whole organism. - Examples include **tetanus and diphtheria vaccines**, which protect against diseases caused by bacterial toxins, not viral infections like chickenpox.

Question 1555: Which antibiotic is Actinomycosis sensitive to?

A. Streptomycin
B. Nystatin
C. Doxycycline
D. Penicillin (Correct Answer)

Explanation: ***Penicillin*** - **Penicillin** is the **antibiotic of choice** for treating Actinomycosis due to the organism's high sensitivity. - Treatment typically involves a **long course** of high-dose penicillin for several months. *Streptomycin* - **Streptomycin** is an **aminoglycoside antibiotic** primarily used for **tuberculosis** and some gram-negative bacterial infections. - It is **not effective** against Actinomyces species. *Nystatin* - **Nystatin** is an **antifungal medication** used to treat **yeast infections**, particularly Candida. - It has **no antibacterial activity** and thus no role in treating Actinomycosis. *Doxycycline* - While **doxycycline** can be used as an **alternative** in patients allergic to penicillin, it is **not the primary choice**. - Its effectiveness is generally less pronounced than penicillin, and it's reserved for second-line treatment.

Question 1556: What is the mechanism of action of quinolones?

A. Inhibit tetrahydrofolate reductase
B. Inhibit DNA gyrase (Correct Answer)
C. Bind to 30S ribosomal subunit
D. Bind to bacterial cell membrane

Explanation: ***Inhibit DNA gyrase*** - Quinolones, particularly **fluoroquinolones**, exert their bactericidal effect by targeting **bacterial DNA gyrase (topoisomerase II)** and **topoisomerase IV**. - This inhibition prevents the uncoiling and replication of bacterial DNA, leading to cell death. *Bind to 30S ribosomal subunit* - This mechanism is characteristic of **aminoglycosides** and **tetracyclines**, which disrupt bacterial protein synthesis. - Quinolones do not interfere with ribosomal function but rather with **DNA replication**. *Bind to bacterial cell membrane* - This is the mechanism of action for **polymyxins** and **daptomycin**, which disrupt the integrity of the bacterial cell membrane. - Quinolones specifically target **intracellular enzymes** involved in DNA handling. *Inhibit tetrahydrofolate reductase* - This enzyme name in the option is technically imprecise; **trimethoprim** actually inhibits **dihydrofolate reductase**, which is part of the **sulfonamide-trimethoprim (Bactrim)** combination. - This pathway is involved in **folic acid synthesis**, crucial for bacterial DNA and RNA production, a mechanism distinct from quinolones.

Question 1557: What is the primary reason for using a combination of four drugs in Anti-Koch's Treatment (AKT) for tuberculosis?

A. To decrease the risk of resistance due to mutation. (Correct Answer)
B. To decrease the risk of resistance due to conjugation.
C. To enhance overall treatment efficacy.
D. To simplify treatment.

Explanation: ***To decrease the risk of resistance due to mutation*** - **Tuberculosis bacteria** can spontaneously develop resistance to a single drug through **random genetic mutations**. - Using multiple drugs simultaneously significantly reduces the probability that a bacterium will spontaneously develop resistance to **all drugs** in the regimen. - This is the **primary rationale** for multi-drug therapy in TB, as emphasized by WHO guidelines. *To decrease the risk of resistance due to conjugation* - **Conjugation** is a mechanism of horizontal gene transfer in bacteria, primarily involving the transfer of plasmids. - While important for antibiotic resistance in some bacteria, it is **not the primary mechanism** of resistance development in *Mycobacterium tuberculosis*. - TB resistance develops mainly through **chromosomal mutations**, not plasmid transfer. *To enhance overall treatment efficacy* - While multi-drug regimens do enhance treatment efficacy by targeting different bacterial populations (actively dividing, slow-growing, dormant), this is a **consequence** of the multi-drug approach. - The **primary reason** for using four drugs specifically is to prevent the emergence of **drug-resistant mutants**. - Enhanced efficacy is achieved *because* resistance is prevented, making this a secondary benefit. *To simplify treatment* - A four-drug regimen actually makes treatment more **complex** due to multiple pills, potential drug interactions, and increased side effects. - The complexity is a necessary trade-off for **resistance prevention** and treatment success.

Question 1558: All are true about ciprofloxacin except?

A. More active at acidic pH (Correct Answer)
B. DNA gyrase inhibition
C. Contraindicated in pregnancy
D. Second generation fluoroquinolone

Explanation: ***More active at acidic pH*** - Fluoroquinolones, including ciprofloxacin, exhibit **reduced antibacterial activity in acidic environments**. Their efficacy is generally better at **neutral or alkaline pH**. - This is clinically relevant as fluoroquinolones may have **reduced effectiveness in acidic sites** like the stomach or acidic urine. - The optimal antibacterial activity occurs at physiological or slightly alkaline pH. *DNA gyrase inhibition* - Ciprofloxacin, like other fluoroquinolones, exerts its antibacterial effect by inhibiting **bacterial DNA gyrase (topoisomerase II)** and **topoisomerase IV**. - This inhibition prevents DNA replication and repair, leading to bacterial cell death. *Contraindicated in pregnancy* - Ciprofloxacin is generally **contraindicated in pregnancy** due to concerns about potential harm to the developing fetus, particularly effects on **cartilage development**. - However, it may be used in specific, life-threatening situations if the benefit outweighs the potential risk. *Second generation fluoroquinolone* - Ciprofloxacin is classified as a **second-generation fluoroquinolone**. - This class includes agents with improved activity against Gram-negative bacteria and some atypical organisms.

Question 1559: Which of the following drugs is not used for prophylaxis against malaria?

A. Doxycycline
B. Artesunate (Correct Answer)
C. Mefloquine
D. Chloroquine

Explanation: Artesunate - **Artesunate** is a **fast-acting artemisinin derivative primarily used for treating acute malaria infections, especially severe or complicated cases**, and is not recommended for prophylaxis [4]. - Its short half-life and rapid clearance make it unsuitable for preventing malaria, as it would require frequent dosing, which is impractical for long-term prevention. *Doxycycline* - **Doxycycline** is an **antibiotic** that can be used for malaria prophylaxis, particularly in areas with **chloroquine-resistant P. falciparum** [1]. - It works by inhibiting protein synthesis in the parasite and must be taken daily, starting before travel and continuing for a period after returning [1]. *Chloroquine* - **Chloroquine** was historically a cornerstone for malaria prophylaxis due to its effectiveness and low cost, particularly in areas with **sensitive Plasmodium species.** [2] - However, widespread **chloroquine resistance**, especially in **P. falciparum**, has limited its current use for prophylaxis to specific regions where resistance is not prevalent [2]. *Mefloquine* - **Mefloquine** is an **antimalarial drug** commonly used for prophylaxis, especially in areas with **chloroquine-resistant P. falciparum** [2], [3]. - It is known for its convenience of once-weekly dosing but can have significant **neuropsychiatric side effects**, limiting its use in some individuals [3].

Question 1560: What is the primary purpose of administering a multidrug regimen for tuberculosis (TB) treatment?

A. To prevent the development of drug resistance (Correct Answer)
B. To provide broad-spectrum coverage
C. To minimize side effects
D. To ensure treatment adherence

Explanation: ***To prevent the development of drug resistance*** - Using multiple drugs simultaneously targets different bacterial pathways, reducing the likelihood of *Mycobacterium tuberculosis* evolving resistance to any single drug [1] - This strategy ensures that even if a few bacteria are naturally resistant to one drug, other drugs in the regimen will eliminate them [1] - This is the fundamental principle behind multidrug TB therapy as per WHO guidelines *To provide broad-spectrum coverage* - TB treatment uses multiple drugs specifically against *Mycobacterium tuberculosis*, not for broad-spectrum coverage [2] - These regimens are tailored to the known characteristics of TB bacteria, not to cover a wide range of pathogens [2] - The drugs used (rifampicin, isoniazid, pyrazinamide, ethambutol) are relatively specific for mycobacteria [2] *To minimize side effects* - Administering multiple anti-TB drugs actually increases the risk of cumulative side effects due to drug interactions and individual toxicities [3] - Each drug has its own toxicity profile (hepatotoxicity, optic neuritis, hyperuricemia) [3] - While side effects are monitored, minimizing them is not the primary reason for multidrug approach *To ensure treatment adherence* - Treatment adherence refers to the patient's consistent use of prescribed medications, not the number of drugs - A complex multidrug regimen can actually make adherence more challenging - Directly Observed Therapy (DOT) is often needed to improve adherence with multidrug regimens

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

Want unlimited practice?

Get full access to all questions, explanations, and performance tracking.

Start For Free