Antimicrobial Agents — MCQs

Antimicrobial Agents Indian Medical PG Practice Questions and MCQs

Question 1351: Which of the following drugs is not given in the blister pack in NLEP?

A. Rifampicin
B. Dapsone
C. Clofazimine
D. Minocycline (Correct Answer)

Explanation: ***Minocycline*** - **Minocycline** is an alternative drug in **multibacillary (MB) leprosy** but is not part of the standard blister pack regimen provided by the National Leprosy Eradication Programme (NLEP). - It is typically reserved for cases where patients cannot tolerate or are resistant to standard drugs like **clofazimine**. *Rifampicin* - **Rifampicin** is a crucial component of the standard **multidrug therapy (MDT)** blister packs for both **paucibacillary (PB)** and **multibacillary (MB)** leprosy [1], [2]. - It is a highly potent bactericidal drug against **Mycobacterium leprae** and is given once a month [2]. *Dapsone* - **Dapsone** is a daily medication included in the standard **MDT blister packs** for both **PB** and **MB** leprosy [1]. - It is a bacteriostatic drug that inhibits folate synthesis in the bacteria. *Clofazimine* - **Clofazimine** is an essential daily component of the **MDT blister pack** for **multibacillary (MB) leprosy** [1], [2]. - It has both **bacteriostatic** and anti-inflammatory properties and is responsible for skin discoloration.

Question 1352: Continued suppression of bacterial growth after antibiotic levels have fallen below the Minimum Inhibitory Concentration (MIC) is known as?

A. Time dependent killing
B. Sequential blockade
C. Concentration dependent killing
D. Post antibiotic effect (Correct Answer)

Explanation: ***Post antibiotic effect*** - The **post-antibiotic effect (PAE)** refers to the continued suppression of bacterial growth after antibiotic levels have fallen below the **Minimum Inhibitory Concentration (MIC)**. - This phenomenon allows for less frequent dosing while maintaining efficacy, which is important for drug scheduling. *Time dependent killing* - **Time-dependent killing** means that the duration for which the antibiotic concentration stays above the **MIC** is the most important factor for efficacy, not necessarily the peak concentration. - Antibiotics with this characteristic, such as **beta-lactams**, often require frequent dosing or continuous infusion. *Sequential blockade* - **Sequential blockade** occurs when two drugs act on consecutive steps in a metabolic pathway, leading to a synergistic effect that results in enhanced microbial killing. - A classic example is the combination of **sulfamethoxazole and trimethoprim**, which inhibit different enzymes in the folic acid synthesis pathway. *Concentration dependent killing* - **Concentration-dependent killing** indicates that the rate and extent of bacterial killing increase as the antibiotic concentration rises, particularly when it exceeds the **MIC**. - Antibiotics like **aminoglycosides** exhibit this effect, often benefiting from high peak concentrations to maximize efficacy.

Question 1353: Which of the following statements about terbinafine is FALSE?

A. It is effective in onychomycosis
B. Its activity is restricted to dermatophytes (Correct Answer)
C. It is unlikely to produce anti-androgenic side effects
D. It is not effective in cryptococcal meningitis

Explanation: Its activity is restricted to dermatophytes - This statement is **false** because while **terbinafine** is highly effective against **dermatophytes**, it also exhibits activity against other fungi like *Candida* species and *Aspergillus*, especially at higher concentrations. - Its broad-spectrum fungicidal action differentiates it from azoles, which are primarily fungistatic and target a wider range of fungi. *It is effective in onychomycosis* - **Terbinafine** is a **first-line treatment** for **onychomycosis** due to its high efficacy, ability to penetrate nail tissue, and fungicidal action against dermatophytes [1]. - It accumulates in the nail plate, providing sustained antifungal effects, leading to high cure rates in fungal nail infections. *It is unlikely to produce anti-androgenic side effects* - **Terbinafine** acts by inhibiting **squalene epoxidase** in the fungal cell membrane, which is distinct from the **cytochrome P450 enzymes** involved in **steroid hormone synthesis**. - Therefore, it does not interfere with human androgen receptors or steroid synthesis pathways, making **anti-androgenic side effects** highly unlikely, unlike some azole antifungals. *It is not effective in cryptococcal meningitis* - **Terbinafine** primarily concentrates in **skin, nails, and adipose tissue**, and its **penetration into the central nervous system (CNS)** is poor. - **Cryptococcal meningitis** requires antifungal agents that can effectively cross the blood-brain barrier, such as **amphotericin B** and **flucytosine**, which terbinafine cannot achieve [1].

Question 1354: Which of the following statements regarding drugs used in leprosy is false

A. Clofazimine may cause changes in skin colour
B. Single intramuscular injections of acedapsone maintain inhibitory levels of dapsone in tissues for up to 3 months
C. Clofazimine should not be given to patients who are intolerant to dapsone or who fail to improve during treatment with dapsone (Correct Answer)
D. Monthly doses of rifampicin delay the emergence of resistance to dapsone

Explanation: ***Clofazimine should not be given to patients who are intolerant to dapsone or who fail to improve during treatment with dapsone*** - This statement is false because **clofazimine** is a crucial second-line agent used in multidrug therapy for leprosy, particularly effective in cases of **dapsone resistance** or intolerance. - Its inclusion is vital for treating patients who do not respond to dapsone or develop adverse reactions to it, making it an alternative, not a contraindication. *Clofazimine may cause changes in skin colour* - This statement is true; **clofazimine** is known to cause dose-dependent **reddish-brown discoloration** of the skin and conjunctiva. - This side effect is usually reversible but can take several months or even years to fade after discontinuation of the drug. *Single intramuscular injections of acedapsone maintain inhibitory levels of dapsone in tissues for up to 3 months* - This statement is true. **Acedapsone** is a prodrug that is hydrolyzed to **dapsone** and can be administered via a single intramuscular injection to provide sustained therapeutic levels for up to three months. - This property makes it useful for long-acting treatment, though it is not widely used due to the preference for oral multidrug regimens. *Monthly doses of rifampicin delay the emergence of resistance to dapsone* - This statement is true. **Rifampicin** is a potent bactericidal drug against *Mycobacterium leprae*, and its inclusion in multidrug therapy, even in monthly doses, helps **prevent the development of resistance** to other drugs like dapsone. - Multidrug therapy is crucial for effective treatment and minimizing resistance in leprosy.

Question 1355: Production of inactivating enzymes is an important mechanism of drug resistance for all of these antibiotics EXCEPT

A. Quinolone (Correct Answer)
B. Penicillin
C. Chloramphenicol
D. Aminoglycoside

Explanation: ***Quinolone*** - The primary mechanisms of resistance to **quinolones** involve mutations in the **gyrase** and **topoisomerase IV** enzymes or efflux pump overexpression, rather than enzymatic inactivation of the drug itself. - Unlike other antibiotic classes listed, quinolones are not typically susceptible to bacterial enzymes that degrade or modify their structure. *Penicillin* - **Penicillins** are highly susceptible to inactivation by **beta-lactamase enzymes**, which hydrolyze the beta-lactam ring, rendering the antibiotic ineffective. - This enzymatic degradation is a major mechanism of resistance developed by many bacterial species to penicillin and other beta-lactam antibiotics. *Chloramphenicol* - Resistance to **chloramphenicol** is primarily mediated by the enzyme **chloramphenicol acetyltransferase (CAT)**, which acetylates the drug, preventing its binding to the bacterial ribosome. - This enzymatic modification is a classic example of drug inactivation leading to resistance. *Aminoglycoside* - **Aminoglycosides** are frequently inactivated by a variety of **aminoglycoside-modifying enzymes (AMEs)**, such as acetyltransferases, phosphoryltransferases, and nucleotidyltransferases. - These enzymes add chemical moieties to the aminoglycoside molecule, preventing its binding to the bacterial ribosome and inhibiting protein synthesis.

Question 1356: β-lactamases cause resistance to penicillins and cephalosporins by

A. Breaking one chemical bond (Correct Answer)
B. Inhibiting transpeptidase enzymes
C. Adding amino (NH2) groups to a molecule
D. Breaking two chemical bonds

Explanation: ***Breaking one chemical bond*** - β-lactamases mediate resistance by **hydrolyzing a single amide bond** within the **β-lactam ring** of penicillins and cephalosporins [1]. - This hydrolysis opens the β-lactam ring, rendering the antibiotic **inactive** and unable to bind to **penicillin-binding proteins (PBPs)** [1, 2].*Inhibiting transpeptidase enzymes* - This describes the **mechanism of action of β-lactam antibiotics themselves**, not the mechanism of resistance by β-lactamases. - β-lactam antibiotics work by **irreversibly binding to and inhibiting transpeptidase enzymes** (PBPs), which are crucial for bacterial cell wall synthesis.*Adding amino (NH2) groups to a molecule* - This mechanism, known as **acetylation or adenylation**, is characteristic of resistance to certain other antibiotic classes, such as **aminoglycosides**. - It does not describe how β-lactamases confer resistance to β-lactam antibiotics.*Breaking two chemical bonds* - While β-lactamase action involves breaking a bond, it specifically involves the **hydrolysis of a single amide bond** in the β-lactam ring. - Breaking two distinct chemical bonds is not the mechanism by which β-lactamases inactivate these antibiotics.

Question 1357: Chemoprophylaxis of malaria can be done by all except:

A. Chloroquine
B. Proguanil
C. Mefloquine
D. Primaquine (Correct Answer)

Explanation: ***Primaquine*** - Primaquine is primarily used for **radical cure** of *P. vivax* and *P. ovale* malaria by targeting **hypnozoites** in the liver, and for **terminal prophylaxis (PART)** after exposure ends. - While it can be used for primary prophylaxis in special circumstances, it is **not a first-line choice** for routine traveler chemoprophylaxis due to its short half-life requiring daily dosing, and the risk of **hemolysis in G6PD-deficient individuals**. - Unlike other options listed, primaquine is not routinely recommended as a standard prophylactic agent. *Chloroquine* - Chloroquine remains an effective prophylactic agent for malaria in areas with **chloroquine-sensitive strains**. - Its use has become limited due to widespread **chloroquine resistance**, especially to *P. falciparum*, but it is still used for prophylaxis in sensitive regions. *Proguanil* - Proguanil is commonly used in combination with **atovaquone** (as Malarone) or **chloroquine** for malaria prophylaxis in many regions. - It acts by inhibiting **dihydrofolate reductase**, disrupting parasite DNA synthesis and replication. *Mefloquine* - Mefloquine is an effective prophylactic agent for areas with **multi-drug resistant *P. falciparum***. - It is typically taken once weekly, but its use can be limited by potential **neuropsychiatric side effects**.

Question 1358: Drug of choice for carriers of typhoid is

A. Chloramphenicol
B. Ciprofloxacin (Correct Answer)
C. Co-trimoxazole
D. Clindamycin

Explanation: ***Ciprofloxacin*** - **Ciprofloxacin** is the **drug of choice** for treating **chronic carriers** of **typhoid fever** due to its good penetration into tissues where Salmonella typhi can persist, such as the gallbladder. - It is a **fluoroquinolone**, effective against intracellular bacteria and achieves high concentrations in bile. *Chloramphenicol* - While effective for **acute typhoid fever**, **Chloramphenicol** is generally **not recommended** for treating **carriers** due to its potential for serious side effects, such as **bone marrow suppression** (aplastic anemia). - Its use has largely been replaced by safer and equally effective alternatives for carrier states. *Co-trimoxazole* - **Co-trimoxazole** (trimethoprim-sulfamethoxazole) can be used for **acute typhoid**, but it is **less effective** than fluoroquinolones for eradicating the **carrier state**. - Resistance to co-trimoxazole is also increasing, further limiting its utility for chronic carriers. *Clindamycin* - **Clindamycin** is primarily effective against **anaerobic bacteria** and some gram-positive cocci; it has **no significant activity** against **Salmonella typhi**. - Therefore, it is **not used** in the treatment of typhoid fever or its carrier state.

Question 1359: The following statements are true about DPT vaccine except

A. Whole killed bacteria of Bordetella pertussis has an adjuvant effect
B. Presence of acellular pertussis component increases its immunogenicity (Correct Answer)
C. Aluminium salt has an adjuvant effect
D. Presence of Hemophilus influenza type B component increases the immunogenicity of pertussis component

Explanation: ***Presence of acellular pertussis component increases its immunogenicity*** - This statement is **incorrect** and is the answer to this "except" question. - **Acellular pertussis (aP)** vaccines were developed primarily to **reduce side effects and reactogenicity**, not to increase immunogenicity. - aP vaccines contain purified components (2-5 antigens) compared to whole-cell pertussis (wP) which contains the entire killed bacterium. - **Immunogenicity comparison:** aP vaccines provide **shorter-lived immunity** and may require more boosters compared to wP vaccines, indicating they are not superior in immunogenicity. *Whole killed bacteria of Bordetella pertussis has an adjuvant effect* - **TRUE statement.** Whole-cell pertussis (wP) vaccines naturally possess **adjuvant properties** due to complex bacterial components including lipopolysaccharides (LPS). - These components stimulate strong innate immune responses, leading to robust and longer-lasting immunity, though with more reactogenicity. *Aluminium salt has an adjuvant effect* - **TRUE statement.** Aluminium salts (aluminium hydroxide or phosphate) are standard adjuvants in DPT vaccines. - They enhance immune response through **depot effect** (prolonged antigen release), activation of innate immunity, and recruitment of antigen-presenting cells to the injection site. *Presence of Hemophilus influenza type B component increases the immunogenicity of pertussis component* - **TRUE statement.** When Hib is combined with DPT (forming pentavalent vaccine), each component elicits an **independent immune response** against its specific target. - Hib component does **NOT enhance** the immunogenicity of the pertussis, diphtheria, or tetanus components - they maintain their individual immunogenic properties without mutual enhancement.

Question 1360: Which is the fastest microbicidal agent against M. leprae?

A. Minocycline
B. Clofazimine
C. Rifampicin (Correct Answer)
D. Dapsone

Explanation: ***Rifampicin*** - **Rifampicin** is widely recognized as the most rapidly acting bactericidal drug against **Mycobacterium leprae**, leading to rapid clearance of bacilli. - Its potent and fast action is crucial for reducing the bacterial burden quickly and preventing further transmission of **leprosy**. *Minocycline* - **Minocycline** is an effective anti-leprosy drug, but it is **bacteriostatic** rather than rapidly bactericidal compared to rifampicin. - It is often used as an alternative in cases of **drug resistance** or intolerance to first-line agents but is not the fastest acting. *Clofazimine* - **Clofazimine** is an anti-leprosy drug with **bacteriostatic** and anti-inflammatory properties, but its action is slower than rifampicin. - It is valuable in multi-drug therapy for preventing resistance and managing **lepromatous leprosy**, but not for rapid killing. *Dapsone* - **Dapsone** is a foundational anti-leprosy drug, but its action against **M. leprae** is generally considered **bacteriostatic** and slow. - While essential for long-term treatment, it does not achieve the rapid sterilizing effect seen with rifampicin.

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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