Antimicrobial Agents — MCQs

Antimicrobial Agents Indian Medical PG Practice Questions and MCQs

Question 1651: Maximum sterilizing action is shown by which anti-TB drug?

A. Pyrazinamide
B. Streptomycin
C. INH
D. Rifampicin (Correct Answer)

Explanation: ***Rifampicin*** - Rifampicin has the **maximum sterilizing activity** among all anti-TB drugs, killing semi-dormant and intermittently metabolizing bacilli in various tissue environments. - Its sterilizing effect is crucial for **shortening treatment duration from 12-18 months to 6 months** and preventing relapses. - Rifampicin acts on persisters that other drugs cannot eliminate, making it the most important sterilizing drug in TB therapy. *Pyrazinamide* - Pyrazinamide has **unique sterilizing activity specifically in acidic environments** (pH < 5.5) within macrophages and caseous necrotic lesions. - While highly effective in acidic conditions where other drugs work poorly, its sterilizing action is environment-specific, not the maximum overall. - Its addition to regimens allows treatment shortening from 9 months to 6 months. *Streptomycin* - Streptomycin is an **aminoglycoside** with bactericidal action against actively dividing extracellular M. tuberculosis. - It does not penetrate cells well and has minimal activity against intracellular or dormant bacilli, thus limited sterilizing action. *INH* - Isoniazid (INH) is the most potent **early bactericidal drug** against rapidly multiplying tubercle bacilli. - While highly effective at reducing bacterial load initially, it has limited activity against dormant persisters and thus less sterilizing action compared to rifampicin.

Question 1652: Which of the following drugs inhibits post-translational modification of viral proteins?

A. Indinavir (Correct Answer)
B. Lamivudine
C. Zalcitabine
D. Enfuvirtide

Explanation: ***Indinavir*** - This drug is a **protease inhibitor** that specifically targets the **HIV protease** enzyme [1]. - By inhibiting protease, Indinavir prevents the cleavage of viral polyproteins into functional individual proteins, thereby inhibiting the **post-translational modification** necessary for viral maturation and infectivity [2]. *Enfuvirtide* - Enfuvirtide is a **fusion inhibitor** that prevents HIV from entering CD4+ T-cells by blocking the fusion of the viral envelope with the host cell membrane. - Its mechanism of action is upstream of protein synthesis and modification, directly impacting viral entry, not post-translational processing. *Lamivudine* - Lamivudine is a **nucleoside reverse transcriptase inhibitor (NRTI)** that acts as a chain terminator during reverse transcription. - It interferes with the conversion of viral RNA into DNA, affecting an earlier stage of the HIV life cycle, not post-translational modification. *Zalcitabine* - Similar to Lamivudine, Zalcitabine is also a **nucleoside reverse transcriptase inhibitor (NRTI)**. - It incorporates into the newly synthesized viral DNA strand, causing premature termination because it lacks a 3'-hydroxyl group, thereby inhibiting DNA synthesis rather than protein modification.

Question 1653: Which drug acts by inhibiting oxidative phosphorylation in helminths?

A. Piperazine
B. Niclosamide (Correct Answer)
C. Mebendazole
D. Praziquantel

Explanation: ***Niclosamide*** - This drug uncouples **oxidative phosphorylation** in the mitochondria of helminths, specifically **tapeworms**, depriving them of ATP and leading to their death. - Its mechanism of action prevents the helminth from generating energy, which is crucial for its survival and function. *Piperazine* - This drug acts as a **GABA agonist** in parasitic worms, leading to **hyperpolarization** and paralysis of the helminth muscle. - It does not inhibit oxidative phosphorylation but rather causes **flaccid paralysis**, allowing the worms to be expelled. *Praziquantel* - This anthelmintic agent increases the **permeability of the cell membrane** of schistosomes and other trematodes and cestodes to **calcium ions** [1]. - This influx of calcium causes **tetanic contractions and paralysis** of the worm, leading to its detachment and death. *Mebendazole* - This drug selectively inhibits the synthesis of **microtubules** in intestinal helminths by binding to beta-tubulin. - This action impairs glucose uptake and nutrient absorption, ultimately leading to the **depletion of glycogen stores** and the death of the worm.

Question 1654: Most important side effect of ethambutol is:

A. Hepatotoxicity
B. Renal toxicity
C. Peripheral neuropathy
D. Retrobulbar neuritis (Correct Answer)

Explanation: ***Retrobulbar neuritis*** - **Ethambutol** is known to cause **retrobulbar neuritis**, which presents as **decreased visual acuity**, **red-green color blindness**, and a **central scotoma** [1, 2]. - This side effect is typically **dose-dependent** [1] and can be reversible if detected early and the drug is discontinued. *Hepatotoxicity* - While other anti-tuberculosis drugs like **isoniazid** and **rifampin** are commonly associated with hepatotoxicity, it is **not the primary or most important side effect of ethambutol**. - Ethambutol's impact on the liver is generally minor compared to its ocular effects. *Renal toxicity* - **Renal toxicity** is **not a significant or common side effect** of ethambutol. - Patients with pre-existing renal impairment may require dose adjustments due to altered drug elimination, but the drug itself does not commonly induce renal damage. *Peripheral neuropathy* - **Peripheral neuropathy** is a well-known side effect of **isoniazid**, another anti-tuberculosis drug, particularly in patients with pre-existing risk factors or vitamin B6 deficiency. - It is **rarely associated with ethambutol** and is not considered its primary adverse effect.

Question 1655: Which of the following is not an antifungal drug?

A. Undecylenic acid
B. Ciclopirox
C. Clofazimine (Correct Answer)
D. Ketoconazole

Explanation: ***Clofazimine*** - **Clofazimine** is an **antibiotic** primarily used in the treatment of **leprosy** and occasionally other mycobacterial infections. - It does not have significant antifungal activity and its mechanism involves binding to bacterial DNA and inhibiting growth. *Ketoconazole* - **Ketoconazole** is an **azole antifungal** drug commonly used to treat various fungal infections, including dermatomycoses and systemic candidiasis. - Its mechanism of action involves inhibiting **ergosterol synthesis**, a vital component of fungal cell membranes. *Undecylenic acid* - **Undecylenic acid** is a topical **fatty acid antifungal** derived from castor bean oil, used for superficial fungal infections, especially ringworm. - It works by disrupting the **cell membranes** of fungi, leading to leakage of cellular contents. *Ciclopirox* - **Ciclopirox** is a broad-spectrum **hydroxypyridone antifungal** agent used primarily for topical treatment of dermatophyte, yeast, and other fungal skin infections, as well as onychomycosis. - Its mechanism involves inhibiting the transport of essential substances into the fungal cell and interfering with **DNA, RNA, and protein synthesis**.

Question 1656: A child was admitted to the hospital with Haemophilus influenzae meningitis. Cefotaxime is preferred over ampicillin because...

A. Cefotaxime is more active against H.influenzae with altered penicillin-binding proteins.
B. Cefotaxime is a bactericidal drug.
C. Cefotaxime is more effective against beta-lactamase producing strains. (Correct Answer)
D. Cefotaxime has lower oral bioavailability than ampicillin.

Explanation: ***Cefotaxime is more effective against beta-lactamase producing strains.*** - Many strains of *H. influenzae* produce **beta-lactamase enzymes** that inactivate ampicillin. - Cefotaxime, a **third-generation cephalosporin**, is stable against these beta-lactamases, making it effective for treating resistant strains. *Cefotaxime is more active against H.influenzae with altered penicillin-binding proteins.* - While altered **penicillin-binding proteins (PBPs)** can confer resistance to some beta-lactams, the primary mechanism of ampicillin resistance in *H. influenzae* is **beta-lactamase production**. - Cefotaxime's effectiveness is mainly due to its resistance to beta-lactamase degradation, not necessarily superior activity against *all* forms of altered PBPs. *Cefotaxime is a bactericidal drug.* - Both **cefotaxime** and **ampicillin** are bactericidal drugs that kill bacteria. - This characteristic alone does not explain why cefotaxime is preferred over ampicillin for *H. influenzae* meningitis in an era of antibiotic resistance. *Cefotaxime has lower oral bioavailability than ampicillin.* - This statement is generally true; cefotaxime is typically administered parenterally for serious infections like meningitis. - However, **oral bioavailability** is not the reason for preferring cefotaxime over ampicillin for *H. influenzae* meningitis; efficacy against resistant strains is the critical factor.

Question 1657: Resistance to zidovudine develops due to:

A. Mutation at reverse transcriptase (Correct Answer)
B. Increased efflux of the drug from inside the cell
C. Increased metabolism of the drug
D. Decreased zidovudine 5'-triphosphate formation

Explanation: ***Mutation at reverse transcriptase*** - Zidovudine is a **nucleoside reverse transcriptase inhibitor (NRTI)** that acts by inhibiting **HIV reverse transcriptase**. - **Mutations in the reverse transcriptase gene** can alter the enzyme's structure, preventing zidovudine from binding effectively or allowing the enzyme to efficiently excise the incorporated zidovudine, thereby leading to drug resistance. *Increased efflux of the drug from inside the cell* - While efflux pumps can cause drug resistance in some contexts (e.g., cancer chemotherapy), they are **not the primary mechanism** of zidovudine resistance in HIV. - Zidovudine resistance is predominantly driven by **viral enzyme modifications**. *Increased metabolism of the drug* - Enhanced host metabolism of zidovudine would reduce its availability, but this is **not a primary mechanism** of virus-mediated resistance. - Viral resistance mechanisms typically involve alterations in the **target protein** (reverse transcriptase) or reduced drug activation. *Decreased zidovudine 5 triphosphate formation* - Zidovudine must be **phosphorylated intracellularly to its active triphosphate form** to inhibit reverse transcriptase. - While impaired phosphorylation could theoretically reduce drug efficacy, zidovudine resistance is predominantly linked to **reverse transcriptase mutations** rather than issues with host cell phosphorylation.

Question 1658: All of the following drugs are commonly used in regimens against H. pylori, except:

A. Bismuth Subcitrate
B. Omeprazole
C. Oxytetracycline (Correct Answer)
D. Amoxicillin

Explanation: ***Oxytetracycline*** - **Oxytetracycline** is a tetracycline antibiotic, but it is **not a first-line drug** for *H. pylori* eradication and is less commonly used in standard regimens compared to other tetracyclines like doxycycline. - Its use in *H. pylori* eradication is limited due to **higher rates of resistance** and it is often considered only in **salvage therapies** if other regimens fail. *Amoxicillin* - **Amoxicillin** is a cornerstone **beta-lactam antibiotic** frequently included in **triple and quadruple therapy** regimens for *H. pylori* due to its efficacy and good safety profile. - It is active against *H. pylori* by **inhibiting cell wall synthesis**. *Bismuth Subcitrate* - **Bismuth subcitrate** is a key component of **bismuth quadruple therapy (BQT)**, which is recommended for *H. pylori* eradication, especially in areas with high clarithromycin resistance. - It has both **direct bactericidal effects** against *H. pylori* and helps **disrupt bacterial adhesion** to the gastric mucosa. *Omeprazole* - **Omeprazole** is a **proton pump inhibitor (PPI)** and is an essential component of nearly all *H. pylori* eradication regimens (triple, quadruple, concomitant). - PPIs **reduce gastric acid secretion**, which not only helps to heal peptic ulcers but also **enhances the stability and efficacy of antibiotics** against *H. pylori*.

Question 1659: 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 1660: What is the drug of choice for treating Mycoplasma infections?

A. Cefadroxil
B. Erythromycin
C. Chloramphenicol
D. Azithromycin (Correct Answer)

Explanation: ***Azithromycin*** - **Azithromycin** is a macrolide antibiotic, which is highly effective against **Mycoplasma species** due to its mechanism of inhibiting bacterial protein synthesis. - **Mycoplasma** lacks a cell wall, rendering beta-lactam antibiotics ineffective; macrolides like azithromycin are therefore a primary choice. *Cefadroxil* - **Cefadroxil** is a **cephalosporin antibiotic** (a type of beta-lactam), which targets bacterial cell wall synthesis. - **Mycoplasma** species intrinsically lack a cell wall, making them resistant to all beta-lactam antibiotics, including cefadroxil. *Erythromycin* - **Erythromycin** is an older macrolide antibiotic that, while effective against Mycoplasma, has a **less favorable pharmacokinetic profile** (e.g., more frequent dosing) and often more gastrointestinal side effects compared to newer macrolides like azithromycin. - **Azithromycin** is generally preferred due to its **once-daily dosing** and better tolerability, making it the more common "drug of choice" among macrolides for Mycoplasma infections. *Chloramphenicol* - **Chloramphenicol** is a broad-spectrum antibiotic that inhibits bacterial protein synthesis, but it is typically reserved for **life-threatening infections** due to its potential for serious side effects, such as **bone marrow suppression** (aplastic anemia). - While active against Mycoplasma, its toxicity profile makes it **not a first-line drug of choice** when safer and equally effective alternatives like macrolides are available.

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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