Antimicrobial Agents — MCQs

Antimicrobial Agents Indian Medical PG Practice Questions and MCQs

Question 1561: Which of the following drugs does not inhibit bacterial protein synthesis?

A. Aminoglycosides
B. Chloramphenicol
C. Clindamycin
D. Sulfonamides (Correct Answer)

Explanation: ***Sulfonamides*** - Sulfonamides do **NOT** inhibit bacterial protein synthesis; instead, they inhibit **folic acid synthesis**. - They act as **competitive inhibitors** of dihydropteroate synthase, an enzyme involved in the synthesis of dihydrofolic acid. - Folic acid is essential for nucleotide synthesis and DNA replication, making sulfonamides bacteriostatic agents that work through a completely different mechanism than protein synthesis inhibitors. *Aminoglycosides* - Aminoglycosides bind to the **30S ribosomal subunit**, causing misreading of mRNA and premature termination of protein synthesis. - This leads to the production of **abnormal and non-functional proteins**, ultimately killing the bacterial cell. *Chloramphenicol* - Chloramphenicol binds to the **50S ribosomal subunit**, thereby inhibiting the peptidyl transferase enzyme. - This prevents the formation of **peptide bonds** between amino acids, effectively blocking protein elongation. *Clindamycin* - Clindamycin also binds to the **50S ribosomal subunit**, specifically at the P-site. - It interferes with the **translocation step** of protein synthesis, preventing ribosomal movement along the mRNA.

Question 1562: As per RNTCP guidelines, Multi drug resistance (MDR) TB is defined as resistance to:

A. Rifampicin
B. Rifampicin, isoniazid and ethambutol
C. None of the above
D. Rifampicin and isoniazid (Correct Answer)

Explanation: ***Rifampicin and isoniazid*** - According to **RNTCP guidelines** (now NTEP), **MDR-TB** is specifically defined as tuberculosis that is resistant to at least both **rifampicin** and **isoniazid**. - These two drugs are the **most potent first-line anti-TB medications**, and resistance to both significantly complicates treatment. *Rifampicin* - While resistance to **rifampicin alone** is a serious concern, it is classified as **rifampicin-resistant TB (RR-TB)**, not full **MDR-TB**. - **MDR-TB** requires resistance to at least two key first-line drugs. *Rifampicin, isoniazid and ethambutol* - Resistance to **rifampicin**, **isoniazid**, and **ethambutol** would be a form of **MDR-TB** (as it includes resistance to rifampicin and isoniazid), but it is a more extensive form of resistance. - The minimum definition of **MDR-TB** focuses on the two most crucial first-line drugs. *None of the above* - This option is incorrect because there is a specific definition for **MDR-TB** that aligns with one of the provided choices. - The guidelines clearly define **MDR-TB** based on resistance to specific drugs.

Question 1563: Which antibiotic is primarily associated with Red man syndrome?

A. Polymyxin B
B. Rifampicin
C. Vancomycin (Correct Answer)
D. Teicoplanin

Explanation: **Vancomycin** - **Red man syndrome** is a well-known **infusion-related reaction** primarily associated with vancomycin. - It occurs due to the **rapid infusion** of vancomycin, leading to non-immunologic **mast cell degranulation** and histamine release. - Characterized by flushing, pruritus, and erythema of the upper body and face. - **Prevention:** Slow infusion rate (over 60 minutes or longer) and/or premedication with antihistamines. *Polymyxin B* - Polymyxin B is associated with **nephrotoxicity** and **neurotoxicity**, not typically with Red man syndrome. - It can cause **histamine release** leading to flushing and itching, but this is less common and less severe than with vancomycin. *Rifampicin* - Rifampicin is primarily known for causing **orange-red discoloration** of bodily fluids (e.g., urine, tears, sweat) and can cause **hepatotoxicity**, but not Red man syndrome. - It works by inhibiting bacterial **RNA synthesis** and is used to treat tuberculosis and other mycobacterial infections. *Teicoplanin* - Teicoplanin is a **glycopeptide antibiotic** similar to vancomycin, but it has a **significantly lower incidence** of Red man syndrome. - While it can cause some **infusion-related reactions**, they are generally milder and less frequent compared to vancomycin.

Question 1564: Sulphonamide injection causes decrease in folic acid by?

A. Inhibition through competition with substrate (Correct Answer)
B. Inhibition without competition
C. Inhibition through a different site
D. Inhibition that does not involve the active site

Explanation: ***Inhibition through competition with substrate***Sulphonamides are **structural analogs of p-aminobenzoic acid (PABA)**, a substrate crucial for dihydropteroate synthase [1, 2]. They competitively inhibit this enzyme, which synthesizes **dihydrofolic acid**, a precursor to **tetrahydrofolic acid (THF)**, thereby reducing folic acid production in bacteria [1, 2].*Inhibition without competition*This typically refers to **non-competitive inhibition**, where the inhibitor binds to an allosteric site and changes the enzyme's conformation, regardless of substrate concentration. Sulphonamides, however, specifically compete with PABA at the active site of **dihydropteroate synthase**.*Inhibition through a different site*This describes **allosteric inhibition** or non-competitive inhibition, where the inhibitor binds to a site other than the active site. Sulphonamides do not work through an allosteric mechanism; they directly interfere with the binding of PABA at the enzyme's active site.*Inhibition that does not involve the active site*This is another way to describe **non-competitive** or **allosteric inhibition**, where the inhibitor binds elsewhere on the enzyme, altering its function without directly blocking the active site. Sulphonamides' mechanism is distinct, as they closely resemble the natural substrate and directly compete for the active site of **dihydropteroate synthase**.

Question 1565: Which of the following is not used as treatment for lymphatic filariasis -

A. DEC
B. Albendazole
C. Ivermectin
D. Praziquantel (Correct Answer)

Explanation: ***Praziquantel*** - **Praziquantel** is primarily an **anthelmintic drug** effective against **schistosomiasis** and **tapeworm infections**. - It does not have a significant role in the treatment of **lymphatic filariasis**. *Ivermectin* - **Ivermectin** is one of the **mainstays** of treatment for **lymphatic filariasis**, particularly in combination therapies. - It works by paralyzing and killing **microfilariae**, reducing their numbers in the bloodstream. *DEC* - **Diethylcarbamazine (DEC)** is a **highly effective antifilarial drug** used to kill both **microfilariae** and **adult worms** in lymphatic filariasis. - It is often used in mass drug administration programs and for individual treatment. *Albendazole* - **Albendazole** is an **anthelmintic drug** often used in combination with **Ivermectin** or **DEC** for the treatment of **lymphatic filariasis**. - It helps to kill **microfilariae** and has some macrofilaricidal effects, reducing the viability of adult worms.

Question 1566: In a child admitted with Haemophilus influenzae meningitis, cefotaxime was started instead of ampicillin. Which of the following is the likely reason for this?

A. It is cheap
B. H. influenzae strains known to produce Beta lactamase (Correct Answer)
C. Easier to give
D. H. influenzae strains known to have altered penicillin binding protein

Explanation: ***H. influenzae strains known to produce Beta lactamase*** - Many *H. influenzae* strains, particularly in meningitis, produce **beta-lactamase enzymes** that inactivate ampicillin. - **Cefotaxime**, a third-generation cephalosporin, is **stable against beta-lactamases**, making it an effective empirical treatment. *Easier to give* - The route and ease of administration are generally **not the primary factors** in choosing between ampicillin and cefotaxime in a severe infection like meningitis. - Both medications can be administered intravenously, which is standard for meningitis treatment. *It is cheap* - While cost is a consideration in healthcare, **efficacy and clinical outcome** for a life-threatening condition like meningitis take precedence over cost. - Treatment choices are primarily driven by **antimicrobial susceptibility patterns** and patient safety. *H. influenzae strains known to have altered penicillin binding protein* - While **altered penicillin-binding proteins (PBPs)** can lead to resistance in some bacteria (e.g., *Streptococcus pneumoniae*), it is **not the predominant mechanism of resistance** to ampicillin in *H. influenzae*. - **Beta-lactamase production** is the much more common and significant reason for ampicillin resistance in *H. influenzae*.

Question 1567: What is the recommended duration of erythromycin for the treatment of diphtheria?

A. 3 days
B. 7 days
C. 14 days (Correct Answer)
D. 30 days

Explanation: ***14 days*** - The recommended duration for **erythromycin** in treating diphtheria is **14 days** to ensure eradication of *Corynebacterium diphtheriae* and prevent toxin production. - This duration helps to eliminate the carrier state and reduce the risk of transmission to others. *3 days* - A 3-day course of antibiotics is **insufficient** for the complete eradication of *Corynebacterium diphtheriae* in diphtheria. - Such a short duration would likely lead to **treatment failure** and persistence of the infection. *7 days* - While 7 days is a common antibiotic duration for some infections, it is generally considered **too short** for diphtheria treatment. - A 7-day course may not fully eliminate the bacteria, potentially leading to **relapse** or a prolonged carrier state. *30 days* - A 30-day course of erythromycin for diphtheria is **unnecessarily long** and could increase the risk of side effects and antibiotic resistance. - The goal is eradication without excessive drug exposure, which 14 days achieves effectively.

Question 1568: Anaerobic bacteria are intrinsically resistant to which of the following antibiotics?

A. Beta-lactams (e.g., penicillin)
B. Aminoglycosides (e.g., gentamicin) (Correct Answer)
C. Chloramphenicol (broad-spectrum antibiotic)
D. Metronidazole (used for anaerobic infections)

Explanation: ***Aminoglycosides (e.g., gentamicin)*** - Anaerobic bacteria lack the **oxygen-dependent transport systems** necessary to take up aminoglycosides into the bacterial cell. - This results in **intrinsic resistance** because the drug cannot reach its intracellular ribosomal target. *Beta-lactams (e.g., penicillin)* - While some anaerobes (like certain *Bacteroides* species) can be resistant to specific beta-lactams due to **beta-lactamase production**, it's not an intrinsic resistance across all anaerobes to all beta-lactams. - Many anaerobes are **susceptible to penicillin**, especially those that do not produce beta-lactamase, or to beta-lactamase inhibitors combination drugs. *Chloramphenicol (broad-spectrum antibiotic)* - Chloramphenicol is effective against many anaerobic bacteria by inhibiting **protein synthesis**. - Anaerobes are generally **susceptible** to chloramphenicol, and it is not an antibiotic to which they are intrinsically resistant. *Metronidazole (used for anaerobic infections)* - Metronidazole is a **prodrug** that requires anaerobic conditions to become activated. - It is highly effective against most obligate anaerobes and is a common choice for treating **anaerobic infections**, indicating susceptibility, not intrinsic resistance.

Question 1569: Which of the following antimalarials is a slow-acting schizonticide?

A. Artemether
B. Mefloquine
C. Pyrimethamine (Correct Answer)
D. Quinine

Explanation: ***Pyrimethamine***- Pyrimethamine is a **folate antagonist** [1, 2] that acts as a **slow-acting schizonticide** [1, 3], primarily inhibiting dihydrofolate reductase in the parasite [1].- Due to its slow onset [1], it is typically used in combination with other faster-acting antimalarials, such as sulfadoxine, for treatment or prophylaxis [1].*Artemether*- Artemether is an **artemisinin derivative**, known for its **rapid action** and potent effect against all erythrocytic stages of *Plasmodium falciparum*.- It is a **fast-acting schizonticide** that causes widespread damage to parasite membranes and proteins.*Mefloquine*- Mefloquine is an antimalarial drug recognized for its **long half-life** and efficacy against multidrug-resistant *Plasmodium falciparum*.- While effective, it is considered of **intermediate speed** compared to the rapid action of artemisinins or the very slow action of drugs like pyrimethamine.*Quinine*- Quinine is a **fast-acting schizonticide** that is effective against the asexual erythrocytic forms of *Plasmodium* parasites.- Although potent, its use is limited by potential adverse effects such as **cinchonism** and a short half-life requiring frequent dosing.

Question 1570: On chronic use, linezolid leads to which of the following?

A. Thrombocytopenia (Correct Answer)
B. Deranged LFT
C. Nephrotoxicity
D. Ototoxicity

Explanation: ***Thrombocytopenia*** - **Linezolid** is known to cause **myelosuppression**, particularly **thrombocytopenia**, with prolonged use (typically >2 weeks). - This adverse effect is usually **reversible** upon discontinuation of the drug. - This is the **most characteristic** and **dose-limiting** hematologic toxicity of chronic linezolid therapy. *Deranged LFT* - While **linezolid** can occasionally cause **elevated liver enzymes**, this is a **less common** adverse effect compared to myelosuppression. - **Thrombocytopenia** is far more characteristic of **chronic linezolid use** and is the primary concern requiring monitoring. - Hepatotoxicity with linezolid is typically mild and less dose-limiting than hematologic effects. *Nephrotoxicity* - **Linezolid** is generally considered to have a low risk of **nephrotoxicity** and does not typically cause significant kidney damage. - **Aminoglycosides** or **vancomycin** are examples of antibiotics more commonly associated with nephrotoxic effects. *Ototoxicity* - **Ototoxicity**, characterized by hearing loss or tinnitus, is not a common or recognized side effect of **linezolid** therapy. - This adverse effect is more frequently associated with drugs like **aminoglycosides** or high-dose **loop diuretics**.

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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