Antimicrobial Agents Practice Questions

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

Rifampicin. ***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.

Q: Which of the following drugs is a prodrug that, after oral administration, is converted to an active agent, penciclovir?

Famciclovir. ***Famciclovir*** - **Famciclovir** is an oral prodrug of **penciclovir**. After absorption, it undergoes first-pass metabolism to penciclovir, its active antiviral form. - **Penciclovir** is effective against **herpes simplex virus (HSV)** and **varicella-zoster virus (VZV)**, inhibiting viral DNA polymerase. *Acyclovir (active drug)* - **Acyclovir** is an active antiviral drug and is well-known for its use directly as an antiviral, not as a prodrug for penciclovir. - Although it is also converted to an active triphosphate form inside infected cells, its structure is distinct from penciclovir. *Ganciclovir (for CMV infections)* - **Ganciclovir** is an antiviral drug primarily used to treat and prevent **cytomegalovirus (CMV)** infections. - It is an active nucleoside analog itself, similar to acyclovir, and is not a prodrug for penciclovir. *Fomivirsen (used for CMV retinitis)* - **Fomivirsen** is an antisense oligonucleotide used specifically for the treatment of **CMV retinitis** in AIDS patients. - Its mechanism of action involves binding to CMV mRNA to inhibit viral protein synthesis, and it is unrelated to penciclovir.

Q: A patient with a confirmed infection by ESBL-producing Klebsiella pneumoniae is being treated with ceftriaxone. What modification is needed in his antibiotic treatment?

Replace ceftriaxone with imipenem. ***Replace ceftriaxone with imipenem*** - **ESBL-producing organisms** (Extended-Spectrum Beta-Lactamase) are resistant to **third-generation cephalosporins** like ceftriaxone [1]. - **Carbapenems**, such as imipenem, are generally the drugs of choice for serious infections caused by ESBL-producing bacteria due to their broad spectrum and stability against ESBL enzymes. *Replace ceftriaxone with a fluoroquinolone* - While fluoroquinolones are broad-spectrum antibiotics, resistance in **Klebsiella pneumoniae** is increasing, and they are not reliably effective against ESBL-producing strains. - Using a fluoroquinolone for a confirmed ESBL infection without susceptibility data could lead to treatment failure and increased resistance. *Continue the same antibiotics at higher doses* - **Ceftriaxone** is a beta-lactam antibiotic that is destroyed by **ESBL enzymes**, rendering it ineffective regardless of the dose. - Increasing the dose of an ineffective antibiotic will not overcome the resistance mechanism and may lead to increased toxicity without clinical benefit. *Replace ceftriaxone with ceftazidime* - **Ceftazidime** is also a **third-generation cephalosporin** and is susceptible to degradation by ESBL enzymes. - Switching from one third-generation cephalosporin to another will not address the underlying **ESBL resistance mechanism**.

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

Indinavir. ***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.

Q: Which of the following is not an antifungal drug?

Clofazimine. ***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**.

Q: Resistance to zidovudine develops due to:

Mutation at reverse transcriptase. ***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.

Q: Most important side effect of ethambutol is:

Retrobulbar neuritis. ***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.

Q: Which drug acts by inhibiting oxidative phosphorylation in helminths?

Niclosamide. ***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.

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

Cefazolin. ***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 1

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

Accepted Answer

Rifampicin

Answer

Pyrazinamide

Answer

Streptomycin

Answer

INH

Question 2

Which of the following drugs is a prodrug that, after oral administration, is converted to an active agent, penciclovir?

Accepted Answer

Famciclovir

Answer

Acyclovir

Answer

Ganciclovir

Answer

Fomivirsen

Question 3

A patient with a confirmed infection by ESBL-producing Klebsiella pneumoniae is being treated with ceftriaxone. What modification is needed in his antibiotic treatment?

Accepted Answer

Replace ceftriaxone with imipenem

Answer

Replace ceftriaxone with a fluoroquinolone

Answer

Continue the same antibiotics at higher doses

Answer

Replace ceftriaxone with ceftazidime

Question 4

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

Accepted Answer

Indinavir

Answer

Lamivudine

Answer

Zalcitabine

Answer

Enfuvirtide

Question 5

Which of the following is not an antifungal drug?

Accepted Answer

Clofazimine

Answer

Undecylenic acid

Answer

Ciclopirox

Answer

Ketoconazole

Question 6

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

Accepted Answer

Cefotaxime is more effective against beta-lactamase producing strains.

Answer

Cefotaxime is more active against H.influenzae with altered penicillin-binding proteins.

Answer

Cefotaxime is a bactericidal drug.

Answer

Cefotaxime has lower oral bioavailability than ampicillin.

Question 7

Resistance to zidovudine develops due to:

Accepted Answer

Mutation at reverse transcriptase

Answer

Increased efflux of the drug from inside the cell

Answer

Increased metabolism of the drug

Answer

Decreased zidovudine 5'-triphosphate formation

Question 8

Most important side effect of ethambutol is:

Accepted Answer

Retrobulbar neuritis

Answer

Hepatotoxicity

Answer

Renal toxicity

Answer

Peripheral neuropathy

Question 9

Which drug acts by inhibiting oxidative phosphorylation in helminths?

Accepted Answer

Niclosamide

Answer

Piperazine

Answer

Mebendazole

Answer

Praziquantel

Question 10

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

Accepted Answer

Cefazolin

Answer

Ceftriaxone

Answer

Cefoxitin

Answer

Cefepime

Antimicrobial Agents — MCQs

Antimicrobial Agents — MCQs

On this page

Practice by Chapter

Want unlimited practice?