Antimicrobial Agents — MCQs

Antimicrobial Agents Indian Medical PG Practice Questions and MCQs

Question 1661: Which of the following is a first-generation cephalosporin used for surgical prophylaxis?

A. Ceftriaxone
B. Cefoxitin
C. Cefazolin (Correct Answer)
D. Cefepime

Explanation: ***Cefazolin*** - **Cefazolin** is a **first-generation cephalosporin** routinely used for **surgical prophylaxis** due to its effective coverage against common skin flora like *Staphylococcus aureus* and streptococci. - Its **longer half-life** allows for less frequent dosing pre-operatively, making it practical for preventing surgical site infections. *Ceftriaxone* - **Ceftriaxone** is a **third-generation cephalosporin** with a broader spectrum of activity, including good coverage against many gram-negative bacteria, but it is not typically the first choice for routine surgical prophylaxis. - It is more commonly reserved for treating serious infections such as **meningitis**, **gonorrhea**, and complicated intra-abdominal infections. *Cefoxitin* - **Cefoxitin** is a **second-generation cephalosporin** known for its excellent activity against **anaerobic bacteria**, in addition to gram-positive and some gram-negative organisms. - While it can be used for surgical prophylaxis in procedures with **high anaerobic risk** (e.g., colorectal surgery), it is not a first-generation cephalosporin. *Cefepime* - **Cefepime** is a **fourth-generation cephalosporin** with a very broad spectrum of activity, including excellent coverage against **Pseudomonas aeruginosa** and improved activity against gram-positive bacteria compared to third-generation cephalosporins. - It is reserved for severe infections, such as **febrile neutropenia** and hospital-acquired pneumonia, and is not generally used for routine surgical prophylaxis.

Question 1662: Which of the following drugs causes flaccid paralysis of ascaris?

A. Ivermectin
B. Albendazole
C. Piperazine (Correct Answer)
D. Pyrantel pamoate

Explanation: ***Piperazine*** - Piperazine acts as a **GABA agonist**, leading to **hyperpolarization** of the *Ascaris* muscle cells. - This hyperpolarization causes muscular paralysis, making the worms unable to maintain their position and thus they are expelled. *Albendazole* - Albendazole primarily works by inhibiting **microtubule polymerization** in the parasitic worms, which impairs glucose uptake and energy metabolism. - This leads to the **slow death** of the worms, rather than acute flaccid paralysis. *Pyrantel pamoate* - Pyrantel pamoate induces a **nicotinic acetylcholine receptor agonism** in nematodes, causing **spastic paralysis** [1]. - The worms become irreversibly paralyzed in a contracted state, unlike the flaccid paralysis caused by piperazine. *Ivermectin* - Ivermectin works by binding to **glutamate-gated chloride channels** in invertebrate nerve and muscle cells. - This binding leads to an increase in chloride ion permeability and **hyperpolarization** of the neuronal membrane, causing paralysis and eventual death of the parasite.

Question 1663: Treatment of whooping cough is

A. Rifampicin
B. Tetracycline
C. Erythromycin (Correct Answer)
D. Ampicillin

Explanation: ***Erythromycin*** - **Macrolide antibiotics** like erythromycin are the **first-line treatment** for whooping cough (*Bordetella pertussis*), especially when administered early in the catarrhal stage. - They work by **eradicating the bacteria**, **reducing the duration of infectiousness**, and can lessen the severity and duration of symptoms if given promptly. *Rifampicin* - **Rifampicin** is primarily used in the treatment of **tuberculosis** and other mycobacterial infections, not whooping cough. - Its mechanism of action involves inhibiting bacterial RNA synthesis, which is not the preferred strategy for *Bordetella pertussis*. *Tetracycline* - While **tetracyclines** are broad-spectrum antibiotics, they are generally **not recommended for whooping cough** due to potential side effects, particularly in young children (e.g., tooth discoloration). - Macrolides are superior and safer alternatives for *Bordetella pertussis* infection. *Ampicillin* - **Ampicillin** is a **beta-lactam antibiotic** commonly used for various bacterial infections, but it is **ineffective against *Bordetella pertussis***. - *Bordetella pertussis* shows resistance to penicillins and other beta-lactams, likely due to **beta-lactamase production** and the organism's location in the respiratory tract where beta-lactams may not achieve adequate bactericidal concentrations.

Question 1664: Which antibiotic acts by inhibiting protein synthesis?

A. Cefotetan
B. Doxycycline (Correct Answer)
C. Ciprofloxacin
D. Oxacillin

Explanation: ***Doxycycline*** - **Doxycycline** is a **tetracycline antibiotic** that inhibits bacterial protein synthesis by binding reversibly to the **30S ribosomal subunit**, preventing the attachment of aminoacyl-tRNA. - This action effectively blocks the elongation of the peptide chain, halting bacterial growth. *Cefotetan* - **Cefotetan** is a **second-generation cephalosporin** that inhibits bacterial cell wall synthesis. - It does this by binding to and inactivating **penicillin-binding proteins (PBPs)**, which are essential for peptidoglycan cross-linking. *Ciprofloxacin* - **Ciprofloxacin** is a **fluoroquinolone antibiotic** that inhibits bacterial **DNA synthesis**. - It targets **DNA gyrase (topoisomerase II)** and **topoisomerase IV**, enzymes crucial for DNA replication, transcription, and repair. *Oxacillin* - **Oxacillin** is a **beta-lactam antibiotic** (specifically a penicillinase-resistant penicillin) that, like cephalosporins, inhibits bacterial **cell wall synthesis**. - It achieves this by binding to and inactivating **penicillin-binding proteins (PBPs)**, thereby interfering with peptidoglycan cross-linking.

Question 1665: Which of the following bacteria are not affected by streptogramins?

A. Legionella
B. E. coli (Correct Answer)
C. Staphylococcus aureus
D. M. pneumoniae

Explanation: ***E. coli*** - Streptogramins (e.g., quinupristin/dalfopristin) are primarily active against **Gram-positive bacteria**. - ***E. coli*** is a **Gram-negative bacterium** with intrinsic resistance to streptogramins due to poor penetration through the outer membrane. - This makes E. coli the most clearly **NOT affected** by streptogramins among the options. *Staphylococcus aureus* - **Streptogramins** are highly effective against **Gram-positive bacteria**, including **Staphylococcus aureus**. - They are particularly useful for treating **multidrug-resistant strains like MRSA** and vancomycin-resistant Enterococcus faecium. - This is one of their primary clinical indications. *Legionella* - **_Legionella_** species are **atypical intracellular bacteria** that cause Legionnaires' disease. - While streptogramins have some intracellular penetration, they have **limited and unreliable activity** against Legionella. - Standard treatment uses macrolides or fluoroquinolones, NOT streptogramins. *M. pneumoniae* - **_Mycoplasma pneumoniae_** is an **atypical bacterium** lacking a cell wall that causes atypical pneumonia. - Streptogramins have **limited clinical activity** against M. pneumoniae and are NOT used as standard therapy. - Preferred treatments include macrolides, tetracyclines, or fluoroquinolones.

Question 1666: Which of the following drugs is not effective against methicillin-resistant Staphylococcus aureus (MRSA)?

A. Ciprofloxacin
B. Cefaclor (Correct Answer)
C. Cotrimoxazole
D. Vancomycin

Explanation: ***Cefaclor*** - **Cefaclor** is a second-generation cephalosporin, and like most beta-lactam antibiotics, it is **ineffective against MRSA** due to the presence of the **mecA gene** which codes for an altered penicillin-binding protein (PBP2a). - MRSA's resistance mechanism renders beta-lactams, including cefaclor, unable to bind effectively to their target, thus inhibiting their antibacterial action. *Cotrimoxazole* - **Cotrimoxazole** (trimethoprim-sulfamethoxazole) is often effective against MRSA, especially for **skin and soft tissue infections**, making it a common choice for outpatient treatment. - It works by inhibiting two consecutive steps in the bacterial **folic acid synthesis pathway**, preventing nucleic acid and protein synthesis. *Ciprofloxacin* - **Ciprofloxacin**, a fluoroquinolone, can be effective against certain strains of MRSA, although **resistance rates have increased**, limiting its broad utility. - Its mechanism involves inhibiting **bacterial DNA gyrase** and topoisomerase IV, essential enzymes for DNA replication. *Vancomycin* - **Vancomycin** is a cornerstone in the treatment of **severe MRSA infections**, particularly in intravenous formulations for systemic infections. - It inhibits **bacterial cell wall synthesis** by binding to the D-Ala-D-Ala terminus of peptidoglycan precursors.

Question 1667: All the following statements are true regarding beta-lactams except:

A. Methicillin is not orally bioavailable and is given parenterally.
B. Imipenem should be given with cilastatin
C. Meropenem does not require cilastatin for protection against renal toxicity.
D. Aztreonam shows cross-reactivity with cephalexin. (Correct Answer)

Explanation: ***Aztreonam shows cross-reactivity with cephalexin.*** - **Aztreonam** is a monobactam with a distinct chemical structure from other beta-lactams, resulting in a different **allergy profile** and lack of significant **cross-reactivity** with other beta-lactams, including cephalosporins like cephalexin. - Its unique structure also means it is specifically active against **gram-negative bacteria** and generally resistant to common beta-lactamases that deactivate other beta-lactams. *Imipenem should be given with cilastatin* - **Imipenem** is rapidly metabolized in the renal tubules by **dehydropeptidase-1**, leading to reduced antibiotic concentrations and potential for nephrotoxicity. - **Cilastatin** is a dehydropeptidase inhibitor that prevents the breakdown of imipenem, ensuring adequate drug levels and reducing renal damage. *Methicillin is not orally bioavailable and is given parenterally.* - **Methicillin** is an acid-labile penicillin, meaning it is extensively degraded by stomach acid when taken orally, leading to poor absorption. - Due to its instability in acidic environments, methicillin must be administered **parenterally (intravenously or intramuscularly)** to achieve therapeutic concentrations. *Meropenem does not require cilastatin for protection against renal toxicity.* - **Meropenem** is another carbapenem, but it is much more stable to metabolism by **renal dehydropeptidase-1** compared to imipenem. - This inherent stability eliminates the need for co-administration with **cilastatin** to prevent its degradation and protect against nephrotoxicity.

Question 1668: All of the following are true about rifabutin except which of the following?

A. It has lesser incidence of drug interactions as compared to Rifampicin
B. It is more effective against MAC as compared to Rifampicin
C. It is more effective for newly diagnosed TB as compared to Rifampicin (Correct Answer)
D. It has longer half life than Rifampicin

Explanation: ***It is more effective for newly diagnosed TB as compared to Rifampicin*** - This statement is **false**. Studies have shown that **rifabutin** and **rifampicin** have **similar efficacy** in treating newly diagnosed tuberculosis when used in standard regimens. - Rifabutin is primarily used as an **alternative to rifampicin** in specific situations, such as significant drug interactions or intolerance, rather than being inherently more effective. *It has lesser incidence of drug interactions as compared to Rifampicin* - **Rifabutin** is a **less potent inducer of cytochrome P450 enzymes** (CYP3A4) compared to rifampicin. - This property leads to a **lower incidence of significant drug-drug interactions** with other medications, particularly antiretrovirals used in HIV-positive patients. *It is more effective against MAC as compared to Rifampicin* - **Rifabutin** exhibits **superior activity against Mycobacterium avium complex (MAC)** compared to rifampicin. - For this reason, rifabutin is often preferred or recommended for the **prophylaxis and treatment of MAC infections**, especially in immunocompromised individuals. *It has longer half life than Rifampicin* - **Rifabutin** has a significantly **longer half-life** (approximately 45 hours) compared to rifampicin (2-5 hours). - This longer half-life allows for once-daily dosing and potentially more stable drug concentrations, which can be advantageous in certain clinical scenarios.

Question 1669: What is the treatment of choice for Campylobacter infections?

A. Tetracycline
B. Ampicillin
C. Ciprofloxacin
D. Erythromycin (Correct Answer)

Explanation: Erythromycin - **Erythromycin**, a macrolide antibiotic, is the **correct answer among the given options** for *Campylobacter* infections. - It works by inhibiting bacterial protein synthesis at the 50S ribosomal subunit, effectively reducing the duration and severity of symptoms in acute gastroenteritis. - **Modern practice note:** While azithromycin (another macrolide) has largely replaced erythromycin as the **preferred first-line agent** due to better tolerability, once-daily dosing, and fewer GI side effects, erythromycin remains effective and is the best choice listed here [1]. - Macrolides are particularly indicated in severe cases with high fever, bloody diarrhea, or in immunocompromised patients. *Tetracycline* - While tetracyclines have broad-spectrum activity, they are **not first-line treatment** for *Campylobacter* due to increasing resistance rates globally [2]. - Resistance mechanisms include efflux pumps and ribosomal protection proteins. - May still have some role in regions with documented susceptibility, but macrolides are preferred. *Ampicillin* - **Ampicillin** and other beta-lactam antibiotics are **ineffective** against *Campylobacter* species. - *Campylobacter* possesses intrinsic resistance to beta-lactams through beta-lactamase production and altered penicillin-binding proteins. - Cell wall synthesis inhibitors are therefore not useful for this infection. *Ciprofloxacin* - **Ciprofloxacin** and other fluoroquinolones were previously common first-line agents but are **no longer recommended** due to widespread resistance. - Resistance rates exceed 50-90% in many countries, particularly linked to fluoroquinolone use in poultry farming. - Should only be used if susceptibility testing confirms sensitivity, which is rarely practical in acute gastroenteritis.

Question 1670: Drug of choice for rheumatic fever prophylaxis in a penicillin-allergic patient is?

A. Vancomycin
B. Gentamycin
C. Erythromycin (Correct Answer)
D. Clindamycin

Explanation: ***Erythromycin*** - **Erythromycin** is the **recommended alternative** for **rheumatic fever prophylaxis** in patients with **penicillin allergy** according to **American Heart Association (AHA) guidelines**. - Typical regimen: **Erythromycin estolate 250 mg BID** or **erythromycin ethylsuccinate 400 mg BID**. - Effective against **Streptococcus pyogenes** (Group A Streptococcus), the causative organism of acute rheumatic fever. - While macrolide resistance exists in some regions, erythromycin remains the **guideline-recommended choice** when penicillin cannot be used. *Clindamycin* - **Clindamycin** is **not recommended** by major guidelines (AHA, WHO) for routine rheumatic fever prophylaxis. - Lacks adequate evidence and official guideline support for this specific indication. - May be considered in specific cases but is not the standard alternative. *Vancomycin* - **Vancomycin** is reserved for **multidrug-resistant infections** such as **MRSA**. - Not indicated for rheumatic fever prophylaxis due to high cost, need for IV administration, and potential for resistance development. - Reserved for life-threatening infections where other options have failed. *Gentamycin* - **Gentamycin** is an **aminoglycoside** used primarily for **severe gram-negative infections**. - Lacks adequate activity against **Streptococcus pyogenes**. - Not appropriate for rheumatic fever prophylaxis.

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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